If you own a Tag Heuer watch, the chances are you spent a lot of money. A lot of people find it intimidating to change the battery out themselves. They will take it somewhere and spend a lot of money having someone else do it. You can easily change the battery yourself for a fraction of the cost a jeweler will charge.
Adjust the tips of the tool in order to be in the three notches on the back of the watch. Crank it down so it is very tight and squeeze the notches together.
Push the tool against the watch and the watch against the tool at the same time.
Once you have good contact, turn slowly and the back will pop off.
You will see a tiny strap covering the tiny screws. Using the glasses screwdriver unscrew them part of the way. You do not have to totally unscrew them. If you do, it will be hard to get them back in the notches.
Once you loosen the screws, the tiny strap will swing out sideways. Use the small screwdriver to pop the battery out and replace it with the new one.
Put everything back together and you will have a new working watch.
You can buy an inexpensive watch case tool on Ebay. Make sure you have a three tip tool and not a two tip. Use the smallest glasses screwdriver.