I've opened a lot of these. The trick is to figure out which side is
the hook and which side is the loop. Sometimes they reverse as you go
around the unit. The best tool I've found is an old plastic prop for a
model airplane. Work it into the seam and under the female part to pop
it lose. I suspect a guitar pick would also make a great tool. BTW the
usual problem with these units is a broken solder joint where the
battery wire attaches to the main board.
Richard