Run warm water over the dried poop. Start breaking it up as it softens. It may take a few minutes. after all the poop is gone, carefully dry the chick(s) off, and apply vasaline, or olive oil around the vent to help prevent future 'pasty butt'. Check 'em all for the next few days, and repeat as neccessary.
You need to clean it off! The method that worked the best for me was to have a stream of warm water running, put the chick's butt in it and rub gently. You need to keep doing it every time you have this problem as it can kill your chicks.
Don't pull it off though! Make sure it's very soft, then wipe it off. If you pull it off, you can severely injure the chick. Then, as suggested in a previous post, apply vaseline to the vent to keep it moist.
I used mineral oil on my chick's poopy butts! You can get it at the drugstore near medication for constipation and other human poopy problems! I dunked a qtip in the mineral oil and them dapped in on the chick's butt. Then I used my fingers to crumble the dried poop off. I had to do it about twice a day for nearly a week but it worked. Don't pull too hard...the oil makes the poop crumble very easily with some careful "squeezing" I guess is the best way to put it. Eventually the chick's fur falls out and new feathers grow in.