The tensioner pulley is spring loaded. Put a 15 mm socket or wrench on the bolt in the center of the pulley, and turn clockwise to release the tension and remove the old belt. Route the new belt over everything except the tensioner pulley, and use the wrench to re-position the tensioner pulley to where you can re-install the belt. Remove the wrench, and the belt is automatically set to the correct tension. The only tricky part is placing the belt so it is between the wrench and the pulley so you can slip it on when the pulley is re-positioned.
It really is only a 5 minute job.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)