Mirrors that are attached to the wall can be tricky. The first thing is safety, you should have some type of eye and hand protection. You can get an inexpensive pair of rubber dipped gloves that will let you hold onto the mirror while taking it down. The one thing to be careful is using a metal pry bar will either instantly shatter the mirror or push into your sheetrock wall, We will use a flexible putty knife and work our way around all of the edges that you can and try to separate the mirror from the wall. You want to be careful and two people is better than one for holding the mirror and working around it. We have also used a glass suction cup to pull on the mirror (which you could rent from a local equipment rental place. And if it just doesn’t seem to want to budge I would recommend a glass company to come remove it. Even with a professional doing it there is more than likely going to be some wall repair, the silicone or glue usually tears the paper of the drywall, Just want you to be prepared!