Linq DataSet merge

Hi

I have an untyped DataSet with 3 DataTables populated by the appropriate DataAdapters. I have also set up relations between the Tables. I use LINQ to query the DataTables (there is a join involved). The result of my query is displayed in a DataGridView.

 My question is : how to get the DataGridView back into the original DataSet so that it can be updated back to the db?

Please Help.