Gather a small screwdriver, case opener, tweezers, and a replacement battery
Identify the watch case back type: snap-on, screw-back, or battery hatch
Remove the watch strap if it blocks access
Open the case back carefully with the proper tool
Note the battery model number and orientation
Remove the old battery with non-metal tweezers if possible
Check for a battery clip or retaining plate and release it if needed
Insert the new battery in the same orientation
Make sure the battery is seated securely
Reinstall any clip or retaining plate
Inspect and clean the case gasket if present
Close the case back firmly and evenly
Reset the time, date, and any other functions
Test the watch to confirm it is running
Replace or service the gasket if the watch needs water resistance
Take the watch to a professional if the back is difficult to open or the watch is sealed