Place the mirror frame face down on your work surface. Remove or open the frame clips that hold the backing and mirror in place.
Lift out the backing, which is usually made out of cardboard or particle board. Remove the old mirror from inside the frame. Wear gloves to avoid getting cut if the mirror is broken.
Measure the old mirror or the interior of the frame to determine a replacement size. Visit a frame shop to have a mirror cut to the correct size if you have an odd-shaped frame.
Remove any protective covering from the surface of the new mirror. Mirrors may have a thin plastic film stuck to the surface to prevent scratches.
Set the new mirror in the frame, face down. Replace the backing material. If your new mirror is not as thick as the old one, cut a sheet of thin cardboard to the same size as the backing and set it in the frame to fill up the extra space.
Replace or close the frame clips to secure the mirror in place. Flip the frame over and wash the mirror with glass cleaner to remove any smudges before you hang it.