To service the transmission, you need to check the fluid level with a special procedure using a temperature-specific measurement and a filler tube (often accessible after removing the air cleaner). Then, if needed, the fluid is added or removed through this filler tube. This is a more involved process that requires consulting a Ford service manual for your specific van's engine and transmission type (e.g., C6, E4OD). Improperly adding fluid can cause serious damage.
Do not attempt this without the proper tools and knowledge. It's best to have a qualified mechanic perform a transmission fluid and filter service.