The Timex WR 50M is part of the 1440 Sports watch set. It was named the 1440 because every day has 1,440 minutes in it. The Timex WR 50M uses a snap back for the cover plate for the battery. To change the battery you need a precision screwdriver set because there is a very small Phillips screw that holds the battery inside the watch. A regular screwdriver is too big to remove the screw.
Place the face of your Timex watch onto a table covered with a cotton cloth. This allows you to see the back of the watch and it protects the face of the watch from scratches.
Locate the small recess in the raised back cover. The recess is large enough to accept the tip of a flathead precision screwdriver. Place the tip of the screwdriver into the recess and turn the screwdriver sideways to pry the cover off the back of the watch.
Remove the small screw that holds the small round battery inside the watch. Use a Phillip's precision screwdriver. Before lifting the battery out of the watch, take note of the battery face. It will have a (+) or a (-) on the back. When you place the new battery into the watch, make sure the same symbol faces upward.
Place the new battery into the watch and secure it with the Phillip's screw and the precision screwdriver.
Place the back cover onto the Timex watch and flip the watch over so the cover rests on the cotton cloth. Press on the edges around the face of the watch to snap the battery cover back on.