Press and hold the mode button for a few seconds, then, press the mode button again!
The 'set' button then changes the value while the 'start/stop' button will advance to the next thing to set.
Set the Timex 1440 Sports Watch (wr50m) Magnetism watchhold down the "mode" button for 2 seconds. When the digits on the watch face start to blink, then press the "mode" button again. To change the hours, press the "adjust button". [instructions thanks to a comment below]
To turn the alarm off:Press and hold "set."
At the same time press (and hold?) "start/stop."
You will see the alarm bell icon go away.
Also, see wiki.answers post for more details.