If your Silverlight app references services, the endpoints to these services are stored within a config file within the xap bundle - which is usually deployed as .xap file within the ClientBin dir of your web app.
To change the end point urls:
1) Rename the xap file (eg within E:\Demos\MySilverlightApp.Web\ClientBin) to .zip.
2) Unzip it
3) Edit the ServiceReferences.ClientConfig file (to update the endpoint url) and save it.
4) Recreate the zip file
5) Rename the .zip file as .xap
That's it.