To replace the thermostat in a Ford F-150, drain about a gallon of coolant from the radiator to eliminate any chance of spilling. Follow the upper hose from the radiator up to the engine to find the thermostat housing.
Remove the upper radiator hose and any other coolant hoses. They should slip off when the clamps are loosened. When the coolant hoses are removed, take off the thermostat housing by removing the bolts that hold it in place. Make sure to remember where the thermostat is positioned, as the new one needs to be placed in the same spot.
Remove any debris that has built around the gasket, and block the coolant passage off with a clean rag. The block where the housing is mounted also needs to be cleaned, and should be dry before installing the new parts. The thermostat housing also should be cleaned with brake cleaner, a gasket scraper and emery cloth.
When all of the surfaces are cleaned, place the new thermostat into the housing, taking care not to put it in backwards. Remove the rag that blocks the coolant passage and ensure all surfaces are completely dry. Reinstall the housing with a new gasket, then tighten down the bolts until they are tight. Reinstall the hoses, and refill the radiator. Start the truck and allow it to heat up, then allow it to cool by shutting it off. Check the coolant level and add more if necessary.