It's not clear whether you want to do this client-side or server-side, but with either option the technique is pretty similar.  You'll want to remove the row from the rows collection, and then insert it into the rows collection at the appropriate index.  Here's a link to the client-side API's for the rows collection:

http://help.infragistics.com/Help/NetAdvantage/NET/2007.3/CLR2.0/html/WebGrid_Rows_Object_CSOM.html

-Tony

You'll want to use the insert method for the RowsCollection.