Belt came off going down the road

Well, my belt came off driving down the road today. I got it back on, but its about 1/8" off the tensioner. I put it on centered, and when it starts it slips off a little...

So what do I need to do? Belt is only 3months old. do I need to replace the tensioner? How much are they, and how hard is it to install? Thanks!

sounds like the tensioner is screwed up to me