Gather a small flathead screwdriver, case knife, tweezers, and the correct replacement battery
Place the watch face down on a soft cloth
Open the case back by removing screws or gently prying the snap-back cover
Note the battery type and orientation before removing it
Remove the old battery carefully with tweezers or a non-metal tool
Insert the new battery in the same orientation
Make sure the battery is seated securely
Refit the case back and press it closed or reinstall the screws
Check that the watch starts running
Reset the time and date if needed