In most cases it really is just as simple as twisting it back. It's just that some are more stubborn than others.
Start by stringing the bow and lay it on a table or flat surface. Gradually twist the limb in the opposite direction, and keep working at it. Leave it strung up and repeat the process from time to time. Eventually, most twists will work out that way.
Remember that it probably took a long time to put the twist in it (usually be the old "step through" method of stringing) It might take some time to work it back out.
Sometimes it takes more than that though, and there are various methods to work it out, including heat. But that can be dangerous if too much heat is introduced. It could result in the glue failing and the entire limb delaminating. Extreme caution is advised!
Most of the time simple is better, and working the twist out by hand is the prefered method. Although it may take some time and effort, is much safer for the limb.