This is most commonly caused by a faulty blower motor resistor. The high speed is controlled by a high speed relay while the lower speeds are controlled by a resistor that regulates voltage to the blower motor.
It is fairly easy to replace. It is located next to the blower motor underneath the right side of the dash on passenger side. Remove the harness connector to the resistor, remove the 2 retaining screws and pull out old resistor. Install new one in reverse:
Were you able to find the resistor underneath the dash ok? If you have more questions on it, feel free to ask, if not, please click the accept button