There are 3 ways
1) Can be pried off with a very small screw driver.
2) It can be screwed off. Like the lid on a peanut butter jar. Need special tool to do it.
3) There may be 3 to 5 screws on the back that can be removed.
This gives you access to the battery inside.
Note:
Not sure where you are located, but just take your watch to the
jewelry counter at Walmart and they will do it for you. Battery about $8 Watch how they do it, and you'll be able to do in the future!