Ask a friend to help you. Removing heavy objects is always safer when two or more people work together.
Put on a pair of gloves with a rubber grip. Instruct your friend to do the same. Mirrors are slick and the rubber grip helps you keep hold of the mirror while removing it from the wall.
Place a stepladder on the ground underneath one side of the mirror, if necessary. Instruct your friend to stand on the opposite end of the mirror.
Pull one corner of the mirror's frame away from the wall far enough to peer behind it and see the wall hook to which the mirror is attached.
Grasp the top of the mirror and lift it upward until the wire on the back of the mirror lifts off the wall hook. While you lift the mirror from the top, your friend must lift the mirror from the opposite side at the bottom.
Pull the mirror carefully away from the wall. Step down from the stepladder carefully. Do not let go of the mirror.
Turn the mirror reflective side up. One hand should be on each side of the mirror with your friend holding the other side. Carry the mirror to its new location or to a safe storage area.