If the backlight of your digital watch weakens, the readout fades or the screen just goes blank, you probably need a new battery. You should expect a watch to need a battery replacement every few years. To replace the "button battery" yourself, you'll need to know the 3- or 4-digit number that specifies its size.
Read the back of the watch. The battery identification number may be specified there. Otherwise, proceed to the next step.
Search for your owner's manual. It may be model-specific and tell you what size is needed. Otherwise, proceed to the next step.
Search for the model's manual online by entering the brand name and model number in a website search engine. If none come up in the search that are model-specific and contain the battery size, proceed to the next step.
Remove the back of the watch to look at the numbers on the battery directly. You may need a knife to wedge the back off, since the gap between the watch back and the rest of the body is usually very slender. A screwdriver will almost never be narrow enough to fit into the gap. Look for an indent under the watch back and insert the knife there. Work the knife around the edge, prying the back away. Don't press the knife into the watch; otherwise, it could slip and cut the hand holding the watch.