Gather a case back opener, small flat tool, soft cloth, and replacement battery
Place the watch face down on a soft cloth
Inspect the case back for screws, notches, or a snap-off edge
If there are screws, remove them with the correct small screwdriver
If there is a snap-off back, insert a case knife or small flat tool into the notch
Gently twist or pry to lift the case back
If there is a screw-down back, use a case opener to turn it counterclockwise
Lift off the case back carefully
Note the battery type and position before removing it
Use a non-metal tool to release the battery clip if present
Remove the old battery
Insert the new battery in the same orientation
Refit the battery clip if present
Check that the gasket is seated properly
Replace the case back
Press it down firmly or screw it back on securely
Reinstall any screws if removed
Set the time and date
Test the watch to confirm it is working