Chargers are often sealed to prevent what you are trying to do.
However, if it's screwed together, you may find there are additional screws under the various labels or rubber feet. Other than that, it may just be the internal clips, which you need to carefully locate and release with a thin screwdriver or cover puller.
Repairs aren't generally recommended unless you understand psu's.
Easier fix is to buy a replacement; either the same one or a universal one from somewhere like maplin.
