Very difficult to do. You probably won't be able to reuse the grommet once you remove it. It will take a special.punch and some expertise to reinstall it. There are a lot of liitle parts in the forearm cowl.
If you can avoid removing it, you're a lot better off.
Otherwise, get the 391 manual written by Seamus O'Casaides sold in the classified section on Shotgunworld.com. You should be able to buy the grommet from Coles gunsmithing.