Dim r() As DataRow = SalesDataSet.SalesDetailsRow.Select("SalesID = " & rdr("SalesID"))
The previous code gets rows in the dataset that have the "SalesID" (primary Key) = SalesID retrieved by the reader (rdr("salesID")). And since this is Primary key so r() will get only one row. So r(0) is the only row in the r() collection as it's a zero based index. Finally , as follows you can update each field in this row (r(0)).
r(0)("ProductName") = "exist22"
r(0)("Qty") += 1
r(0)("Price") = 10
r(0)("Amount") = 10
If you still got problems please paste you code here again and state the problem.