It should be relativly simple, unless someone has really cranked the cap nut up tight with a pipe-wrench. Remember that you are actually running the nut out to lock the barrel in place, so you need to tighten the nut, ie; turn it righthand(screwing it down the threads, towards the magazine tube) to be able to turn the barrel, to remove it.
Once you screw the nut in, even just part way, you'll hear the clicks, you should be able to twist the barrel to the left, (left to loosen), and pull it out. Then if you want to clean and lube the magazine tube, unscrew the cap right off the tube. Watch out for the spring and magazine limiting plug, as they will spring right out once you loosen the cap off.