my f150 won,t change to heat from a/c

Check the simplest things first. With engine hot touch hose entering the heater control valve. It should be hot. Now check the hose after the valve it should also be hot. If not, the valve may be bad. Now touch the inlet hose at the firewall, it should be hot. If the outlet house is cold/warm when inlet hose hot, then the heater core is plugged.

Both hot

If both hoses are hot to the touch then the problem is with the blend door or the actuator. They are in the dashboard. Normally the dash has to come out to repair.