If you make a change to the ds, then you have to reassign the ds into the session variable. then you have to rebind the ds to a grid, for example, to see the changes. In what event? What ever event is appropriate, or just create your own sub, or class and do it there.
Changes to db. Many ways. Use the ds.UPdate(), loop throug the ds if necessary. It depends on what you are doing and when you are doing it