1998 Chevy Venture

If you have access to a lift you can change them from underneathe the van.
If not, remove the air cleaner and air snorkel from the engine. Use the room you've just created to reach around to the other side of the engine and remove the last spark plug wire. Change that spark plug. Reinstall the wire and then go to the next spark plug. The plug closest to the front (under the alternator) can be accessed from the passengers' side.
Only remove one spark plug wire and plug at a time to reduce the chance of getting them mixed up during assembly.