Cambiar dinamicamente valores del archivo app.config

Cambiar en tiempo de ejecucion los valores de UserName y Password antes de llamar al servicio en visual basic.NET

el servicio es este

https://e-factura.sunat.gob.pe/ol-ti-itcpfegem/billService?wsdl, el cual genera un app.config

la seccion a cambiar es esta, el servicio no genera esta seccion pero se coloca manualmente
adjunto el archivo completo

<client>
        <endpoint address="https://www.sunat.gob.pe:443/ol-ti-itcpfegem/billService"
          binding="basicHttpBinding" bindingConfiguration="BillServicePortBinding"
          contract="ServiceFacturacion.billService" name="BillServicePort">
          <headers>
            <wsse:Security mustUnderstand="0" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
              <wsse:UsernameToken Id="ABC-123">
                <wsse:Username>xxxxxxxxx</wsse:Username>
                <wsse:Password>xxxxxxxxxx</wsse:Password>
              </wsse:UsernameToken>
            </wsse:Security>
          </headers>
        </endpoint>
      </client>
    </system.Servicemodel>

*********************************
codigo completo app.CONFIG
*********************************

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <system.diagnostics>
        <sources>
            <!-- This section defines the logging configuration for My.Application.Log -->
            <source name="DefaultSource" switchName="DefaultSwitch">
                <listeners>
                    <add name="FileLog"/>
                    <!-- Uncomment the below section to write to the Application Event Log -->
                    <!--<add name="EventLog"/>-->
                </listeners>
            </source>
        </sources>
        <switches>
            <add name="DefaultSwitch" value="Information" />
        </switches>
        <sharedListeners>
            <add name="FileLog"
                type="Microsoft.VisualBasic.Logging.FileLogTraceListener, Microsoft.VisualBasic, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL"
                initializeData="FileLogWriter"/>
            <!-- Uncomment the below section and replace APPLICATION_NAME with the name of your application to write to the Application Event Log -->
            <!--<add name="EventLog" type="System.Diagnostics.EventLogTraceListener" initializeData="APPLICATION_NAME"/> -->
        </sharedListeners>
    </system.diagnostics>
    <system.serviceModel>
        <bindings>
            <basicHttpBinding>
                <binding name="BillServicePortBinding" closeTimeout="00:01:00"
                    openTimeout="00:01:00" receiveTimeout="00:10:00" sendTimeout="00:01:00"
                    allowCookies="false" bypassProxyOnLocal="false" hostNameComparisonMode="StrongWildcard"
                    maxBufferSize="65536" maxBufferPoolSize="524288" maxReceivedMessageSize="65536"
                    messageEncoding="Text" textEncoding="utf-8" transferMode="Buffered"
                    useDefaultWebProxy="true">
                    <readerQuotas maxDepth="32" maxStringContentLength="8192" maxArrayLength="16384"
                        maxBytesPerRead="4096" maxNameTableCharCount="16384" />
                    <security mode="Transport">
                        <transport clientCredentialType="None" proxyCredentialType="None"
                            realm="" />
                        <message clientCredentialType="UserName" algorithmSuite="Default" />
                    </security>
                </binding>
                <binding name="BillServicePortBinding2" closeTimeout="00:01:00"
                    openTimeout="00:01:00" receiveTimeout="00:10:00" sendTimeout="00:01:00"
                    allowCookies="false" bypassProxyOnLocal="false" hostNameComparisonMode="StrongWildcard"
                    maxBufferSize="65536" maxBufferPoolSize="524288" maxReceivedMessageSize="65536"
                    messageEncoding="Text" textEncoding="utf-8" transferMode="Buffered"
                    useDefaultWebProxy="true">
                    <readerQuotas maxDepth="32" maxStringContentLength="8192" maxArrayLength="16384"
                        maxBytesPerRead="4096" maxNameTableCharCount="16384" />
                    <security mode="Transport">
                        <transport clientCredentialType="None" proxyCredentialType="None"
                            realm="" />
                        <message clientCredentialType="UserName" algorithmSuite="Default" />
                    </security>
                </binding>
                <binding name="BillServicePortBinding3" closeTimeout="00:01:00"
                    openTimeout="00:01:00" receiveTimeout="00:10:00" sendTimeout="00:01:00"
                    allowCookies="false" bypassProxyOnLocal="false" hostNameComparisonMode="StrongWildcard"
                    maxBufferSize="65536" maxBufferPoolSize="524288" maxReceivedMessageSize="65536"
                    messageEncoding="Text" textEncoding="utf-8" transferMode="Buffered"
                    useDefaultWebProxy="true">
                    <readerQuotas maxDepth="32" maxStringContentLength="8192" maxArrayLength="16384"
                        maxBytesPerRead="4096" maxNameTableCharCount="16384" />
                    <security mode="None">
                        <transport clientCredentialType="None" proxyCredentialType="None"
                            realm="" />
                        <message clientCredentialType="UserName" algorithmSuite="Default" />
                    </security>
                </binding>
            </basicHttpBinding>
            <customBinding>
                <binding name="BillServicePortBinding1">
                    <textMessageEncoding maxReadPoolSize="64" maxWritePoolSize="16"
                        messageVersion="Soap12" writeEncoding="utf-8">
                        <readerQuotas maxDepth="32" maxStringContentLength="8192" maxArrayLength="16384"
                            maxBytesPerRead="4096" maxNameTableCharCount="16384" />
                    </textMessageEncoding>
                    <httpsTransport manualAddressing="false" maxBufferPoolSize="524288"
                        maxReceivedMessageSize="65536" allowCookies="false" authenticationScheme="Anonymous"
                        bypassProxyOnLocal="false" hostNameComparisonMode="StrongWildcard"
                        keepAliveEnabled="true" maxBufferSize="65536" proxyAuthenticationScheme="Anonymous"
                        realm="" transferMode="Buffered" unsafeConnectionNtlmAuthentication="false"
                        useDefaultWebProxy="true" requireClientCertificate="false" />
                </binding>
            </customBinding>
        </bindings>

      <client>
        <endpoint address="https://www.sunat.gob.pe:443/ol-ti-itcpfegem/billService"
          binding="basicHttpBinding" bindingConfiguration="BillServicePortBinding"
          contract="ServiceFacturacion.billService" name="BillServicePort">
          <headers>
            <wsse:Security mustUnderstand="0" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
              <wsse:UsernameToken Id="ABC-123">
                <wsse:Username>xxxxxxxxx</wsse:Username>
                <wsse:Password>xxxxxxxxxx</wsse:Password>
              </wsse:UsernameToken>
            </wsse:Security>
          </headers>
        </endpoint>
      </client>
    </system.serviceModel>

</configuration>