It cost $17.25 to have POLAR do it. I took mine to several jewelers who all said they couldn't do it because of the specialized tool that is required to get the battery in. The back is easy to take off- but then POLAR has a lock on the putting it back together phase.
Mine was done in June. I didn't try using it until last week (long story- I'd loaned it to a family member who decided it was far too complicated for her and never used it) and it doesn't work. It tells me my resting heart rate is 36-37 and 89 when I'm in the middle of a drenching run. It's going back to POLAR next week
Despite their great reviews, I wouldn't buy a POLAR again. My battery only lasted about 2 years, and I only used it infrequently for spinning classes. I know it's on all the time, but I really didn't get my money's worth.
Mel