Simply rotate the tensioner pulley and slip the old belt off.
Check all of your pulleys and make sure there is no debris on any of them.
Slip the belt over all of the pulleys leaving the tensioner pulley for last.
Make sure the belt is properly on all the other pulleys, rotate the tensioner pulley and slide the new belt over it.
Inspect the entire belt to make sure that it is on every pulley correctly.
You can rotate the tensioner pulley a little to relieve tension enough to make any adjustments to your belt.