Best method I have found is to grip the injector in one hand, pinch/push the o-ring with thumb and forefinger to create a bulge in the o-ring. Use the thumb on the non holding hand and roll the bulged portion of the o-ring up and out of the retaining groove.
I use a bit of vasoline to lube up the o-ring when replacing, not a gob just a smear. Keeps it from pinching/galling when installing. Be certain you seat the injectors fully.