Many Timex watches actually have a triangle stamped into the watch "back" that points to the "lip" into which a small screwdriver tip can pry it off. After replacing the battery, it is often necessary to place the watch into a padded vise to protect the crystal and casing from scratches...and then uniformly tighten the vise until the back snaps back into place. (There was no way thumb pressure could have replaced the last Timex I worked on!) If the case posts that hold the watch band pins prevent the vise jaw from contacting the watch back, a small disc (cut from leather, perhaps) can be placed between the vise jaw and watch back. It takes so much force to re-seat some backs, you may feel you are about to crack the crystal or damage the case rim, so be cautious, but firm!
Mike Mathras, February 2008