updating a database

[C# Programming]

I need help in updating a database using a dataset.

Im tryin to update using a value stored in a variable .

this is the code:

stocktrackerDataSet changes = (stocktrackerDataSet)stocktrackerDataSet.GetChanges();

Usage = retail_ProductsTableAdapter.Update(changes);
MessageBox.Show("updated " + Usage , "Success");
stocktrackerDataSet.AcceptChanges();

Im gettin an argumentnull exception

Please can some one help me ?