Welcome to JA!
Here is the steps to do this job.
Drain the van's engine coolant. With the engine cool and the parking brake applied, remove the radiator cap, place a large container under the radiator drain and remove the drain fitting. After draining the coolant at the radiator, move the container to the engine block and remove its drain plug to drain the remaining coolant.
Remove the coolant reservoir by disconnecting the overflow hose from the radiator filler neck, removing the fasteners for this reservoir and the windshield washer fluid reservoir, lifting both up and separating the coolant reservoir from the other one.
Detach the heater hoses from the heater core tubes at the firewall by loosening their hose clamps. Plug the core tubes with rubber to prevent contaminants from entering.
Reach under the dashboard in the van's passenger compartment and remove the screws for the heater core cover to remove it--the cover should have a half-cylinder shape. Remove the two mounting screws at the rear of the heater core and lift the core out of its housing.
Insert the new heater core into the housing and fasten it with the mounting screws. Connect the core cover back in place.
Unplug and reconnect the hoses to the heater tubes with their clamps. Reconnect any components (like the washer fluid reservoir or coolant reservoir) that you disconnected to access the hoses.
Refill the coolant after replugging the drains at the radiator and engine block. Add the 50/50 mix of water and antifreeze to the radiator until it's full and the reservoir to the lower mark. Run the engine in a ventilated area with the radiator cap off until the upper radiator hose is hot, shut off the engine and let it cool, then add more coolant up to the reservoir's lower mark.
Let me know if i can help more!