Actually changing the belt isn't that bad of a job, although you want to determine why the belt came off. Check all pulleys for that belt path my wiggling and turning them by hand. If they all check out, the tensioner is suspect and should be checked.
Belt change on Series II motor:
There is a catch and searching the forum will help you find more people'* "How to" threads with pictures...here'* the quick explanation.
Jack passenger front of car and support.
Remove plastic splash shield toward the front of the wheel well.
You will see
Remove the nut, stud and spacer behind the mount. (no need to support engine)
Slide old belt out where the spacer is missing, and put new belt back in.
Route and put new belt on pullies once it is on, then put splash shield back in and tire back on.