The Standard PanelView terminals, including the 550, have a bezel and screen that are bonded together. It makes them nearly impervious to delamination or leaking, but it also means you can't take off the top layer and replace it.
The function key labels are actually on a strip of plastic and can be reprinted or customized, but your problem is that the clear plastic above them has become abraded and opaque.
The only method I've found to rehabilitate scratched plastic like the polycarbonate film used on the PanelViews is to get some polishing compound intended for plastic windows (like convertible tops) and apply with generous amounts of elbow grease.
You could send a terminal in for repair, and Rockwell would replace the bezel and screen assembly and keep the logic boards. But it would cost quite a bit.
Try to buff out the scratches on this terminal and when it literally becomes unreadable replace it with one of the Series B PV550's, which have a nice blue/white LED backlight, and keep contact-paper screen protectors on it in the future.