Just as you have your SQL SELECT statement, you must also have an SQL UPDATE statement.
Your OleDbDataAdapter has a SelectCommand set through the OleDbDataAdapter(strSQL
, cnx) constructor; however you don't have a corresponding UpdateCommand assigned.
You'll need something like:
sda.UpdateCommand = new OleDbCommand("UPDATE something SET something = @Something WHERE something = something", conn)
More info on the OleDbDataAdapter.UpdateCommand Property: