If you look very closely at most scopes, about a quarter of an inch from the end is a nearly invisible parting line. On a Leupold its actually right at the gold ring. Everything in front of that line is basically a thread protector with a tiny bit of lock-nut function. To remove it I usually wrap some masking tape; twisting it or bunching it up to supply some grip. Sometimes I've resorted to a strap wrench. If it doesn't break loose fairly easily just quit and give up.
Once the little collar moves it'll spin off like nothing and expose the threaded lens carrier. On the front edge there should be a couple slots on the carrier. I use a hacksaw blade as a tool to turn it, if it doesnt turn with just your fingers. Turning the carrier out shortens the parallax distance. A 1/4 turn is quite a bit and 1/2 turn is a lot. Twist and check, and when you're happy spin the collar back on.
This is a good way re-purpose big-game scopes for rimfires and tweak a big game scope for longer range use. Bear in mind that if you are doing it to fix a previously good scope that changing parallax might have been a symptom of other trouble in the first place.