Firstly i would apply tape to cover the mirror surface this will help prevent it from shattering and sending shards of glass everywhere.
Then remove the top piece of architrave to see if you can work out if it is stuck or just held in place with the architrave.
I would be surprised if it wasn't stuck.
If it is stuck then it will probably be a pig to get it off the wall and very unlikely that it will come off in one piece.
What kind of wall is it attached to?? i.e. brick or stud wall??
If it's on a stud wall and it's stuck then you could cut out the plasterboard around the mirror and lift it off in 1 piece with the mirror still attached.
Most importantly though is safety ....make sure you wear heavy duty leather gloves and safety goggles