That depends on what they are glued on with.
White glue can be loosened with heat - you can actually straighten a bent fin by warming the glue joint over a 100 watt light bulb or, as I have done, with a monocote sealing iron. If you warm it enough, you may actually be able to reposition the fin (if you don't have to actually move it) or pull it off if you do. I think yellow glue is also heat sensitive, but I don't know for sure.
If you used CA or epoxy, I'm not sure there is anything that will dissolve that joint that won't also destroy the fin and/or the body tube. Your best bet might be to cut the fin can off just in front of the fins and build a new fin can. Then use a body tube coupler to connect the new fin can to the rest of the rocket. Of course, by that point, you are almost at building the rocket over again. Also, putting a coupler just in front of the fin can will move the CG backward some, so repairing a marginally stable rocket like this may make it unstable. Check your CP/CG again before flying.