Utilitarianism says that you do what will cause the greatest total amount of happiness/utility for the community as a whole. So you have to sort of know how much the driver knows.
The most likely answer is that you switch to kill the one person. Killing one brings less of a negative impact than killing five and therefore is the right thing to do. If the driver had more time and information, he might want to know who the people are. What if the five are escaped felons and the one is a respected member of the community? Then, killing the five might well be better.
So you have to know (or at least stipulate for the purpose of your answer) what the driver knows. Then, using whatever information he has, determine which course of action creates the most possible happiness/benefit for the community as a whole.