A battery change on a B6 model A4 isn't a particularly easy job, the battery is hidden at the back of the engine bay under plastic covers which have to be removed. Be better to keep the car powered up if possible as you can lose some of the settings such as one-touch lowering of the windows.
If the battery has suddenly started going dead bear in mind these cars have a known fault with the brake light switch failing in the closed position. The brake lights come on with the ignition off so the battery drains down overnight. A replacement switch is about a tenner plus the same to fit.
SSE