You can't. Depending on who made it... it's possible to do it. You'll have to send it off or whatever. If it's a good quality scope it's covered by warranty. If not and it's still a decent scope, the manufacturer will take care of it. You'll pay a minimum of shipping one way more than likely unless it's one of the biggies with a great customer service plan. However, it sounds like you have a cheap POS and you're stuck.