Is the poop still stuck or is it just staining? If it’s just staining I would leave it. Rabbit poop should not get stuck to their fur, but if they are getting poop stuck and pee stains all the time it can be an indication of several things: poor diet, diet litter tray or unsuiitable litter, health problem.
If the poop is no longer stuck, the staining will go away on its own with a bit of time. Rabbits like to keep themselves well groomed. If there is constantly poop getting stuck you need to address the possible causes as well as cleaning it off.
As for how to clean stuck-on poop. A towel in the bottom of a tub with an inch of luke-warm water and sit them in it for several minutes to soften it up. You may have to use your hand to loosen it up and remove it once it’s soaked. Never put more than just their bum and paws in the water as it’s very stressful for them and bathing a rabbit can actually be quite dangerous for them, but butt baths for hygiene are ok.