Changing a battery on a MyTag Classic (CR2016 Battery): 

For the Classic only - you will need a Philips screwdriver sized Philips PH0, PH00 or PH000 to complete the change.  The other units have an opening tool or you can use a simple small flat blade screwdriver. 

Changing a battery on a MyTag Sport (CR2032 Battery)

Changing a battery on a MyTag Style (CR2025 Battery)