Hi,
The battery is soldered in place and the back plate on the watch has a security screw so it would have to go back to Garmin.
This link will take you to the page on Garmins website where you can request a repair
https://my.garmin.com/rma/repair/repairLanding.htm
Is it actually dead orjust running down quickly? I understand that there were some firmware updates in the past, as the watch was showing a low amount of battery life when it was actually OK.
Or it could be a good excuse to buy a new watch www.hrm-reviews.com shows a good range you can buy from their site and use your Amazon account