Pubert
Its fairly easy, all you do is gently prise the gap on the key end of the fob and work your way around the edge as it comes apart (the gap is on the key end on the opposite side to the lock/unlock buttons). Might be worth getting the battery first though. It’s a CR 2032 3 Volt battery and as dietcokeplease says, it's not a rechargeable battery.
Once you've got the back off the fob, you'll need to push a small black plastic clip away from the battery in order to get the battery to pop out (be careful as the plastic clip looks like it could break easily). Then do the reverse to get it back in again.
Hope this helps
Andrew