I see the problem now - so it looks like a number plate holder.
I've not seen one like that before - if the lugs are plastic then you can't bend them (like the back of a picture frame).
So I guess you need to know if the holder is screwed or stuck to the bumper?
I'd try using a magnet to see if there are any screws present
If not, then assume it's stuck on, if so then try to bend a lug at either end and slide the plate out sideways ....