You may find, with time, that your Audi’s computer doesn’t respond to your remote control's signals as well as it used to. However, the remote is expensive, and you don't want to break it while trying to pry it open for a battery replacement. Further complicating the problem is knowing whether your remote takes a CR 2032 type battery or a CR 1620 one. The A4 and A6 generally take the 2032, while the A8 and TT take the 1620, but don't take this as a hard and fast rule for all model years. The general approach to getting access to the batteries is the same, with small differences between car models.
Purchase a new CR 2032 or CR 1620 battery, depending on your Audi's model and year of manufacture. You may want to check your owner's manual, just to be sure. The batteries are available at automotive stores and at Audi dealers.
Press the ridged release button located on the part of the remote control key with the three rings, and pull out the emergency key. The ridges will be located on the opposite side from the rings. Barring this, you can use a butter knife or flathead screwdriver in the gap between the key (or mechanical) portion and the button (electronic) portion of the remote to pry them apart.
Hold the key so that the three buttons face up. You should see more ridges exposed where you've just pulled off the emergency key. Press on the ridges while pulling out the battery carrier. If you don't see the ridges, then insert the butter knife into the gap, on the side, to pry the entire lid off. You actually should be able to do this with your bare hands.
Insert a new battery into the remote control. Make sure the positive (+) symbol faces down.
Slide in the battery carrier back to its original position and reinsert the emergency key.
Drop off the old battery at the store where you bought the new one. They’ll be able to recycle it or dispose of it properly.
The switchblade remote of several Audi models matches that of some VW models. See the References section for a link to a video demonstration of the above procedure involving a VW remote.