how do you replace a gas line thats under the hood that has a leak

It depends on where the leak is to answer what parts and how. You are going to have to locate exactly where, then one of us will be happy to advise you, but I caution you--do not drive the van until it's fixed, there is a very real risk of fire. A Passenger van? What to do if it's loaded with people, kids, and ignites? Don't even want ti think about that. Either find the leak or have it towed to a reputable shop or dealer.