Doing a U-turn is a common phrase used to describe a sudden change of direction. However, there are plenty of synonyms that can be used instead. For example, you could say someone is "making a 180," "turning around," or "reversing course." Other options might include "flipping a U," "doing a hairpin," or "pulling a quickie." Each phrase has slightly different connotations or associations, which may be useful depending on the context in which it's being used. Regardless of which phrase you choose, the idea behind it is the same: someone is changing their path abruptly, often to go in the opposite direction.