Here's the way I do it: Jam some blocks or other support under the oil pan to hold it up, then remove the top engine mount above the belt area. Get a half-inch socket wrench or breaker bar and insert it into the square hole on the tensioner. Push it up against the spring (you may want to extend the handle with a pipe) and the belt will loosen and you can pull it off. Repeat the same move to allow the new belt to be fitted on. You might need a second pair of hands for that.
