First the warning, the grills are made of cheap pot metal and break very easily, I know,I broke one.
Each grill has 6 "ball ended" pins that poke through white nylon inserts. The inserts are popped into holes in the sheet metal. 4 of the pins are located in the corners of the grills, 2 more are near the middle.
When originally installed the nylon inserts were soft & pliable. Now they are hard & un-giving. The pins were originally lubed with white grease. That has either dried or been washed off.
So get out a can of your favorite spray lube, like WD40, locate the pins, and give them a good shot of spray. Next use a hooked tool, or a piece of wire or string looped around a cross piece in the grill, nearest one of the corner pins, and gently pull upwards. The idea is to slowly pull the pin out of the insert. If you encounter resistance apply more lube and try pulling gently again.
If you pull upward with too much force because the grill isn't budging, and then it slips loose all of a sudden, you'll pull the grill up too far and crack it or bend it terminally. So the idea is to slowly pull the pin out of the insert, without using too much force. New nylon inserts are available from the vendors. It is a good idea to replace them once you've removed the grills.
good luck.
George