These instructions for repairing quick connect fitting such for John Guest fitting
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.
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.
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.