You might be alright doing it to a 5g but generally the frame is a structural part of the tank and removing it could lead to failure later.
But, if you wanna give it a go anyway, get a razor blade and slide it around under the frame inside and out. Also if you have a dremel tool or something similar and don't mind destroying the frame you can make some cuts in the corners to make it easier. This work out pretty well for me when I had to remove the frame to replace it on a tank.