(These instructions show repair of a John Guest fitting. Mur-lok fittings are repaired in exactly the same way, but you'll be replacing two O-rings rather than one. See the picture of a Mur-lok double O-ring fitting below.)
Turn off the inlet water and open a faucet downstream of the unit to relieve all pressure. (If it's an RO unit, it's also best to close the valve on top of the storage tank.)
Remove the tube from the fitting by pressing inward on the collet and simultaneously pulling the tube out of the fitting.
Remove the collet from the fitting. Just grasp it with your fingernails and pull it out.
Use a thin bladed screwdriver, a toothpick, tweezers, or some other thin tool to pull the o-ring out of the fitting. Be careful not to damage the fitting, but don’t worry about the o-ring. You're going to throw it away. John Guest fittings have a single o-ring. Mur-lok fittings have two.
Put the new o-ring into the fitting and push it into position with the end of the tube itself. (Don't use a sharp tool that could damage the new o-ring.) The o-ring should be pushed in so that it stands against the back wall of the fitting.
When the o-ring is in place, reinstall the collet by simply pushing it back into the fitting. If the o-ring is slightly out of position, inserting the collet should fix it. You normally won't need to replace the collet, but it doesn't hurt.
Trim 1/2" or so off the end of the tube to give the fitting a new surface to grip. Get a good square cut on the tube. Insert the tube until you feel it hit the bottom of the fitting, then turn the water back on.
(whole house & well units)