Big flat-blade screwdrivers lol. At least that's how I've always done it. Pry up on the gaskets/grommets and take the whole thing out. Then, remove the gasket/grommet from the elbow. To install, put ONLY the gasket/grommet in and then the elbow. Some persuading with a soft-faced hammer may be necessary.
Edit: Actually I just tested both of mine by hand. By prying the elbow at angle, you can get them out. The front one came out with gasket, back gasket would probably need a screwdriver. If the gasket doesn't want to go back in, try getting at least part of it seated and then gently pushing the rest inward with a screwdriver. These things take some practice, but once you get the hang of it, it's pretty easy.