First of all, if the horizontal stab is more or less in line with the wing, the plane will fly fine. The UCD is a fun fly plane that is designed to flop around the sky, hover, do tumbling maneuvers, etc. If it is a little "non-parallel", I would fly it first before I make the change. If you decide you must make this revision, I would pry the stab off the fuselage as best you can, sand/cut the fuselage for a better alignment, and then re-glue it using epoxy glue.