sc_btn_display ('update', 'off') does not work

I have a form (one register) and I would like to disable the update button after the user click on it . I’m using sc_btn_display (‘update’, ‘off’) in afeter update event but it does nothung , the button renmains active . And (worse) I I put sc_exit() in the same event the applications remains in an idle state (loop for ever) .