When to Replace
How do you know that the watch battery needs changing? As a battery loses its charge, you may begin to see the watch functions change. The watch may power off completely, giving you a blank display. It may stop accurately tracking the date or time, or may produce much weaker "Indiglo" light.
For any of these problems, the first thing to do is change the battery. A new battery will generally cost about $5, and should be easily found at your local drug store or RadioShack. Be careful to select the correct battery size, as many watch batteries look very similar, but are not interchangeable. It may indicate on the back of the watch casing which size to use. Usually, Timex watches use a CR 2016 or a CR 2025 watch battery, both available at RadioShack.
Opening the Watch
Opening the battery compartment will require the use of a very small jeweler's screwdriver, or a glasses-repair screwdriver. Sets of these screwdrivers are also available for purchase at RadioShack, but are often available for use on the spot. There will usually be a store set used by employees for just this purpose, and most employees will freely replace the battery for you.
Remove the watch from your wrist and flip it onto its face. Depending on the style of watch, you may need to also remove the watch band to get access to the back of the watch. On the back is a metal plate, held in place with four small screws. Find the screwdriver that best fits these screws, and carefully remove the backing.
Be extra careful to not drop or lose any of the screws. A simple way of keeping them all in the same place is to stick them to the adhesive strip of a Post-it note. The adhesive holds the screws very well, but also lets go with no problem once you need them again. Additionally, the post-it note adhesive won't leave any problematic residue on the screws. If you don't have a Post-it note handy, a magnet will work as well.
Replacing the Battery
Remove the old battery, noting first the position of the positive and negative terminals. Double check the battery size to be sure you have a match. As you put the new battery in the battery compartment, again double check to be sure that the correct positive/negative alignment is used. Once the battery is in place, check the watch to make sure it's powering on.
Close up the back of the watch. Keep track of the extra tiny screws, as they are easy to lose track of and very hard to find once lost. Put in the first screw, but don't tighten it down completely. Repeat this for the remaining three screws. Once all four screws are in place, tighten then down securely.
With the new battery in place, you may need to reset the time and date on the watch. Follow the manufacturer's instructions, and enjoy your watch for several months to come.