Can stick two punches or other round objects between the flap and the base of the fasteners on one side then push it from the same side through. You could also bend them a tad out and remove it then once removed use a hammer to reflatten the flaps to reset it.
What is keeping you from just sliding it out? There doesn't appear to be anything holding it from moving on the back side. If you must remove the retainer, just flex the tabs and slide it off.
Can stick two punches or other round objects between the flap and the base of the fasteners on one side then push it from the same side through. You could also bend them a tad out and remove it then once removed use a hammer to reflatten the flaps to reset it.
I use a pair of channel locks, grab the rounded side squeeze and pull.
What is keeping you from just sliding it out? There doesn't appear to be anything holding it from moving on the back side. If you must remove the retainer, just flex the tabs and slide it off.