WPF

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <connectionStrings>
    <add name="Application.Properties.Settings.DBConnectionString"
      connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\DB.mdb"
      providerName="System.Data.OleDb" />
  </connectionStrings>
</configuration>

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <connectionStrings>
    <add name="Application.Properties.Settings.DBConnectionString"
      connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\DB.mdb"
      providerName="System.Data.OleDb" />
  </connectionStrings>
</configuration>
<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <connectionStrings>
    <add name="Application.Properties.Settings.DBConnectionString"
      connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\DataBase\DB.mdb"
      providerName="System.Data.OleDb" />
  </connectionStrings>
</configuration>

//xml document object
System.Xml.XmlDocument xdoc = new System.Xml.XmlDocument();

FileStream reader = new FileStream(this._XmlFilePath, FileMode.Open, FileAccess.ReadWrite);

xdoc.XmlResolver = null;

//load the xml file
xdoc.Load(reader);

System.Xml.XmlElement Node = (System.Xml.XmlElement)xdoc.DocumentElement.SelectSingleNode("//configuration/connectionStrings/add[@name=\"Application.Properties.Settings.DBConnectionString\"]");
if (Node != null)
{
    //key found, set the value
    Node.Attributes.GetNamedItem("Source").Value = "C:\DataDabe\DB.mdb";

    //finally, save the new version of the config file
    xdoc.Save(reader);
    reader.Close();
}

Warning    1  Could not locate cross-domain policy at 'vuyiswa-pc': Web services might not be accessible at runtime by your Silverlight application. For more details please contact the web server administrator or press F1 after selecting this warning     0   0

An error occurred while trying to make a request to URI 'http://localhost/BLLBusiness/BLLService.svc'. This could be due to attempting to access a service in a cross-domain way without a proper cross-domain policy in place, or a policy that is unsuitable for SOAP services. You may need to contact the owner of the service to publish a cross-domain policy file and to ensure it allows SOAP-related HTTP headers to be sent. This error may also be caused by using internal types in the web service proxy without using the InternalsVisibleToAttribute attribute. Please see the inner exception for more details.