There is tension put on the belt with the timing belt tension pulley. There is a allen head bolt that runs through this pulley to hold it in place. You have to first loosen this bolt, then loosen the tension-adjusting bolt that is sticking up next to the power steerring pump. Once this is all loose, then the belt can be replaced. Each car has a distinct way to tighten the belt (based on marks on the tensioner), so you really should look up your specific car in a manual.

GregS