Updating A Record From A Textbox

i try to create a windows form that act something smilar to a cash machine i managed to enter a mount with my first button then my second button is for withdrawing money it checks the amount in the text amount if it is higher than the amount the small dat hold in the amount it will throw an error message if it is less or equal it write the amount in the text box my goal is to deduct that amount in the textbox amount from the amount already at the database.[code]