The collar attached to the pto shaft slides toward the front of the tractor to release the shaft. I've always found it awkward to remove because, in addition to just being difficult to reach, you have to kind of push on the shaft toward the rear while pulling the collar forward.
Also, I've read posts on here about collars that didn't lock properly when reattaching to the pto shaft and flying off and destroying the trans oil filter, so make sure when you reattach the collar it is locked on by giving it a good pull.