How do you turn off brake light on a 1988 ford econoline van?

What brake light are you talking about?

If you are talking about the rear stoplamps, I would test the switch underneath the dash board attached to the brake pedal.

If you are referring to the red "brake" light in the instrument cluster, I would check the brake fluid to see if you are low, if you are low check brake pads and linings or for signs of a leak. Refill fluid to the line. If the pedal felt weird you should bleed the brakes.