I found the best way to do this is with the BACK SIDE of an exacto knife.
First, flip the picture over and bend the top a little bit. a "bulge" will appear and you can tuck the exacto knife under to loosen it. Do this on both sides. You don't have to pull the side flaps up all the way down to lift the top one.
Once you have the side flaps up, use the knife again to carefully dig under the top flap to make that rise.