To change the battery in a Timex Ironman watch, first remove the back of the watch. The back is held on by tiny screws. Remove the old battery, and insert the new battery. On some models this may require removing an additional screw.
Be careful to note the exact orientation of the sticker covering the battery. The sticker has to be replaced in the exact same orientation after inserting the new battery. If the watch does not resume running after inserting the new battery, it needs to be reset. To reset the watch, short it out by touching the battery and the All-Clear pad simultaneously with a metal object, such as a pair of tweezers.