On the top of the tensioner is a hole in the plastic piece. Stick a big screwdriver in there and rotate the tensioner so that you compress the spring. The belt will slip right off. Release the spring carefully.

When you go back on with a new belt, do the same thing. Rotate the plastic piece, compress the spring, and pop the belt over the idler.

Now is a good time to make sure the idler is in good shape. It should roll smoothly without any rattle. If you can feel any roughness in the bearing, replace it. It tends to let go and do all kinds of damage.

BTW, take a good look at the belt routing and make a diagram of it, if you don't already have one. It's easy to make an error. Don't ask me how I know.