Professional
If its anything like mine (which it should be), the pulley on the far left (when facing ur engine in front of the truck) has a spring mechanism which supplies tension to the belt. There is a "bolt head" (15mm I think) where a socket can attach right in front of the middle of the pulley. With the help of a power bar (depending if the spring is seized or stiff) it should pivot over providing you the slack you need for replacing the belt. Hope this helps!
Veteran
Ditto. It's a 15mm head on the tensioner. With the socket/wrench on the tensioner, pull it towards the driver's side of the engine bay. That will give you the slack you need to install the belt.