After changing the battery in my key fob it does not work

Okay, I am going to assume it worked prior to the battery replacement? Double check that the battery is installed correctly and use a pencil eraser to gently clean the contacts for the battery.
Do you have a second fob to confirm that it is actually a fault with the fob and not something on the van?

If it is a bad fob you will need a dealer or automotive locksmith to try to reprogram the fob. Usually the locksmith will be cheaper, faster and 99% of the time you do not need an appointment. They can also verify that the remote is the problem.

If it is a bad fob you can usually get a new one cheaper online. New is highlighted because used can be a big waste of money when they do not work or the system will not recognize them.