get a bunch of rags, pack them around the master cynlinder, and wiggle and pull the reservoir out of the gromets.
don't touch the brake pedal!
you'll probably have to go to the deal to get new ones, I think I paid $5.
Clean the resevoir, carefully remove the old gomets from the master, be careful not get any crap in the ports. Make sur port are covered with brake fluid to avoid any air getting into the system.
pop in the new gromets, lube the reservoir spouts with clean brake fluid and wiggle and push back in.
Fill up right away with fresh fluid.