I want to replace the drivebelt of my 2005 Acura MDX but I can't figure which one is the tensioner if the upper or the lower?

 

top (closest to you when looking into engine bay standing up) is pulley, thats what you need to release the tension inorder to release your serpentine/drive belt. if you are looking to replace that pulley then you need to remove the whole tensioner assemebly.