Just run their butt under comfortably warm water. Let it loosen up a minute and work it out with your fingers. Be careful to not grab the umbilical by accident. I have used a drop of Dawn dishsoap if it is really bad. I towel the baby off and hold them a minute or two and put them back under the heat lamp. I have heard of people using a hair dryer but I don't as I don't want to overheat them.
The food can cause it and so can stress. If it is just one or two chicks, you may just have to ride it out until they get better on their own. If you have a lot of them doing it, maybe change to a different chick feed. Also they don't need medicated feed until you are getting ready to put them outdoors or if you are bringing a little soil inside to acclimate them slowly. The medicated feed helps them build up immunity to organisms in the soil.
But I have had some that do it for awhile and I just keep them clean so they can poop. If your chick is gaining weight, it will probably be just fine. I think you can buy electrolytes for them as well.
Good luck, I hope it clears up for you.