If you want to change / replace the battery on a VW, Skoda, Audi key, you have to follow these stepes from the video below:
STEP 1: open the key
STEP 2: split the key in half with barehands or with a flat-blade screwdriver. Try to insert the flat screwdriver into the cut out on the blade side and carefully twist. Pull the two sections apart.
STEP 3: put the key blade apart and split the remote key with a more sharpen flat-blade screwdriver
STEP 4: take out the battery using a small flat-blade screwdriver
STEP 5: insert the battery with writing side down
STEP 6: fit that two parts of remote back on
STEP 7: put back the blade