Blower motor resistors in general are irreparable and are best replaced if damaged, corroded or malfunctioning. However, you can perform a few tests on a blower motor resistor to confirm it is indeed the cause of the problem so you don't have to replace it needlessly.
The first test is a visual test. It is often easy to tell if a resistor is bad because of burn marks around the resistor. A burned-out resistor should be replaced with the part recommended by the car manufacturer.
The second test is a multimeter test. Set a multimeter to resistance-measuring mode, clip one of the multimeter's probes to the common point, then use the other probe to test the other points for resistance. If any of the other points displays an open circuit or infinite resistance, replace the blower resistor.
If there's nothing wrong with the blower resistor visually and the resistance readings are normal, then the problem lies somewhere else in the blower system. Other components to check in the system include the blower fuse, the fan relay and the fan motor. Use a multimeter to test these components.
Blower motor resistor failure is often caused by a blower motor drawing too much current. This usually happens when something prevents the fan blades from spinning or when the motor endures too much wear and tear. As such, if a blower resistor gets damaged, it's a good idea to check the blower motor as well.