First, remove the foil and cutters. You will then need to gently pry off the rubberized front cover with a small flat screwdriver or similar object. You will then be able to access and remove the two screws at the bottom. You will then need to pop the case apart by carefully inserting a flat screwdriver at the bottom between the AC port and the case. Once you pop it apart you will need to remove the metal clip at the top that so that the case can be fully seperated.
You will then be able to see the batteries.
For easier replacement, it is sometimes suggested that you use a modified installation technique. This is to break the tabs off of the old batteries rather than unsoldering them from the circuit board.
This allows the old batteries to be removed but leaves the tabs attached to the circuit board. You can then solder the tabs from the new batteries to the old existing tabs
You will find batteries at shaveroutlet.com. See link below. You will need two batteries.
If this is not something you can do yourself, you might want to consider one of the items below which will give you and easier fix