Hello and welcome to JustAnswer!
What are the tire pressures set at?
Were any of them adjusted?
The tire pressures should be set to 36 PSI when cold - not driven for a few hours. If you set the pressures to 34 PSI when the vehicle was warm then they are still low.
If the tire pressures aren't low and causing the light then the first step tin diagnosing the problem is to use a capable scan tool to see what fault codes are stored. Most likely one or more of the pressure sensors are bad and need to be replaced. One could be bad and no longer sending a signal, or one or more may have a low battery. The internal batteries in the sensors have a finite life and the sensors do need to be replaced over time.
Resetting the average fuel mileage reading isn't hard. All you would need to do is press the step button until you are on the average mileage reading and then press the reset button until it resets.