I have replaced the key fob plastic for my '11 S40. It has these type of keys:
https://www.ebay.co.uk/itm/Fits-Volv...72.m2749.l2649
What I did was that first I removed the metal key part and removed the back part (as if replacing the battery).
From that point on there is only one thing holding the key fob together: glue. Yes, it is glued all around and there is no other way to take it apart, but to force it.
I started from the pointy end, inserted a small screwdriver, then a larger one and carefully forced it open.
WARNING: there is a little glass insert inside the pointy end which is used by the car to identify the key. If you damage this, your key is toast. Make sure not to insert the screwdriver more than 1-2 mm-s from the sides.
Take a look at your new one, you will be able to see where this little glass insert goes, so you will avoid damaging it.
Nothing else to be careful about, just use multiple screwdirvers and take your time.
Once done, try it out and only then glue it back together (try to use as lttile glue as possible).