The best thing would be to bath her (luke warm and make sure it doesn’t get in her ears or eyes or nose) and just rub it under water to wash it off
Remember to keep the bath water low though so it doesn’t go anyway near her head! Probably just enough to cover her bum and add a towel underneath in the tub so there’s less slipping going on
And then dry her off with a towel doesn’t have to be perfect since its warm there but she shouldn’t be too damp by the time you’re done
And then it sounds like maybe she has longer fur down there so I’d trim the fur around there to prevent it from getting dirty again!
This will stress her out so make sure you have her favorite treat and pet or cuddle her after she is all dried to help her calm down.
It’s best for her to get it all managed especially if there’s a chance of flystrike
Also you could get a litter cover to help if she still seems to be getting her butt wet, basically a grid (either plastic or metal) over the litter box so the pee just falls through and can’t be sat on (the plastic ones are a bit better for their feet I think)
This especially works if she is already litter trained but tends to sit in her litter to much
Edit: then again, if it’s just a stain and there isn’t any actual pee matting her fur then she’d probably be fine as light furs (especially white) do tend to stain even after cleaned
But if you can’t tell if there’s any pee stuck in there I’d probably bath her anyways