Unpicking a cover stitch use to be a maddening process for me. I would learn to live with a wonky hem or sew a second row because I was so time consuming and difficult to rip out. I experimental for a while and found this method.
You need to start on the underside of where you ended up stitching. The photo shows you where you need to put your seam ripper and cut the 3 threads. Then the whole darn thing unravels. You can see more pics and a video of me doing it here http://www.detectivehoundstooth.com/unpicking/