Spray the area with brake clean so you can see the white sleeve. it's a thick plastic sleave that goes around the fitting. You push that sleeve in so that it will press metal fingers in, that are currently sticking out and holding the fitting in.
Once you have the fitting clean, soak it down with a penetrating oil or wd-40, then take two flat blade screw drivers and carefully work to push the sleeve in. Take your time so you don't wreck it. You should only be able to see about 1/8th or less of the white sleeve when it is fully inserted. Then grab ahold of the line and twist and tug. Hopefully it'll pull free. It's a self sealing fitting so you shouldn't see much fluid come out.