If you are refering to change the connectionstring of an existing Analysis Services data source, you can do this:
* In you are in BIDS (Visual Studio) project, double clic the data source file (.ds), the Data Source designer pops up.
* In project is already deployed, browse using Object Explorer using SQL Server Management Studio: Your SSAS Instance>Your SSAS Database> Data Sources> Your Data Source. Right click Your Data Source, Script Data Source As> Alter
to> New Query Window. Here you have the XMLA script for your data source. Update the <Connectionstring> element with the new connectionstring. Click the execute button (F5) to send updates to the server.
Hope this helps.
Alan Koo Labrin | "Microsoft Business Intelligence y más..."
http://www.alankoo.com