How do I reset my keyless entry on my 2003 chevy express van 2500?

If you have factory remotes, they can only be programmed by with the use of the factory scan tool.
Contact your local dealer service department. They will be able to help with reprogramming.

If you've used the remote(s) with this vehicle before or if you've replace the remote's battery, you may only need resynchronized.
To do the re-synch, park away from anything that may interfer with programming. Stand about 5' from your vehicle and simultaneously press and hold the LOCK and UNLOCK buttons on the transmitter for 7 seconds. If the re-synch is complete, the locks should cycle once to indicate the remote has been "relearned" and the remote should function properly after that.