If its the same as the 2011, use a thin flat blade screwdriver in the slots on the sides where the back meets the front bezel. Pry gently toward the bezel so that the blade lifts the edge of the back plate away from the housing until you hear it release. Do the top and bottom on one side and it should just pop off.
Even doing it right, I broke one tab on my convex mirror, but it holds fine with 3. The larger mirror has 6.
