These instructions for repairing quick connect fitting such for John Guest fitting
Step 1:
Turn off the incoming feedwater inlet line and open a faucet downstream of the unit to relieve the system's internal pressure and also close the valve on top of the storage tank.
Step 2:
Remove the tube from the fitting by pressing inward on the collet and simultaneously pulling the tube out of the fitting.
Step 3:
Remove the collet from the fitting. Just grasp it with your fingernails and pull it out.
Step 4:
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.
Step 5:
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.
Step 6:
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.