Where is the fuel reset button on a 1998 Ford F150?

There's no fuel reset button on a 1998 Ford F-150 (or virtually any vehicle). Fuel economy calculations are handled by the onboard computer, and resetting average fuel economy is usually done through a sequence of steps involving the trip odometer buttons, not a dedicated button. The specific procedure varies slightly depending on the radio/instrument cluster options in your truck. Consult your owner's manual for the precise instructions.