It is very easy to do. You can open the 'holder' using the key itself. Take the key out and on the bottom of the unit is a slot, put the end of the key in the slot and turn. The unit will split and you can then see the battery. When putting a new battery in make sure it is the right way up and there is no film on the underside, then just click the top and bottom together. I would suggest that you use a good make of battery like Duracel.