Polar recommends that the batteries in its heart-rate monitor be replaced by professionals; however, you can save time and money by replacing the battery yourself. Replacing the battery yourself, however, can invalidate the Polar M32 warranty on your watch, which is why Polar strongly recommends that the watch be sent to the company for battery replacement.
Use the jeweller's screwdriver to remove the four small screws located on the back face. Remove the back of the Polar watch to expose the battery.
Write down the numbers on the battery. This information will be necessary to buy a new battery. Polar has multiple models, and different models require different batteries.
Insert a paper clip, or small pin, into the semicircular opening adjacent to the battery; this releases the metal battery cover. Remove the metal cover.
Place the new battery in the battery cradle and press down gently until you hear it click into place. The battery numbers should be facing up.
Replace the metal battery cover.
Replace the back face of the watch using the jeweller's screwdriver and the four small screws.