Chris
If it is anything like a Mio 168 there are 4 tiny philips screws on the rear of the unit which you undo.
ther are then 2 or 3 clips (part of the front cover) which you need to unclip from the base - insert a very fine bladed (precision) screwdriver in the release it.The battery will then be obvious as will the mini connector which just unplugs - plug in your new one and away you go.
Phil92K
June 2007