A heat gun will easily soften the epoxy but like GB says the covering will be trash. Cut some of the covering off from the area adjacent to the stab, straighten the stab out and then patch the covering back.
This is one of the troubles with ARF's. A kit build would have everything installed and straight before the covering process starts. I've used a heat gun to completely remove things that have been epoxied on and needed to be corrected. Like I say with a kit it's no big deal but with a BARF you'll just have to deal with it the best ya can.

When I glue on a horizontal stab I go thru a process that never fails to be dead on.
I set up my fuse on my glass table top with the wing bolted on and propped up by equal lengths of 1/4" X 1/4" balsa CA'd to my glass table top under each wing tip. I then set the wing incidence by leveling the the stab seat. Then I cut another set of balsa sticks the same length as the seat is off from the table. These are also CA'd to the table top at each end of the stab. Then glue the stab on, let it set till the glue sets and it will be dead on. This whole process takes maybe 30 minutes but the end result is worth it.