Hi, Jan. If you can not see any external fasteners, I guess that you have to assume that the mirrors are glued to the wall. The only way that I know to remove them is to try to pop them off and, obviously, this is going to be very dangerous. Put on a long sleeved shirt and long pants, gloves and eye protection. Get a wide putty knife and try to work around the edges. You might have to gently tap on the end of the putty knife. The chances are great that you will break one or more of the panels so try to work from the side. You will probably tear up the face of the drywall also.
You may elect to just cut around the panels and take a section of drywall out and replace it. This might be a little safer but you would still want to take the same kind of precautions. It is difficult to make the decision about which way to go without seeing it.
Please be careful. We need all the posters on this board.