How I change the battery and what is the cost of a battery.

If the shaver still works but just will not hold a charge as long as it should, you can fix this by replacing the batteries. If the shaver is dead and will not run and the green charging light no longer illuminates when you plug it in, you will need to change the whole circuit board.
You will find the batteries and the circuit board with batteries at shaveroutlet.com. You can order which you require by viewing the links below

http://shaveroutlet.com/shaver-replacement-parts/shaver-replacement-battery-aa-nimh-p-108.html

http://shaveroutlet.com/shaver-replacement-parts/braun-printed-circuit-board-with-batteries-p-106.html

To open the shaver, you will need to pry out the two round plastic screw covers on the bottom then remove the screws underneath. You can then remove the outer case by sliding it down and off of the shaver bode. Then remover the black plastic cover inside to access the circuit board and batteris.
If replacing the entire circuit board, you will simply lift it out and replace. If just replacing the batteries, it is 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