When you create the command, you must add the parameters with the values to it before it can execute:
SqlCommand com = new SqlCommand("UPDATE tbl_OpdClaim SET totalamt=(@totalamt ),Balance =(@Balance ) WHERE userid=@ID", con);
com.Paramaters.AddWithValue("@totalamt", myTotalAmount);
com.Paramaters.AddWithValue("@Balance", myNewBalance);
com.Paramaters.AddWithValue("@ID", txtUserId.Text);
Note also that you should parameterize you ID: concatenating strings like that is an invitation to an accidental or deliberate SQL Injection attack.