It is best to test a new heater in a bucket of water first, to make sure it won't heat the water too much.
Once you have done that (I would try for a 24 hour test period, and at least 8 hours), and the heater keeps the water at the correct, stable temperature, you can add it to your betta's tank. Make sure you increase the temperature gradually. No more than one degree per hour, and no more than five degrees per day.
I am assuming you have a thermometer that goes inside the tank, with a suction cup, and an adjustable heater (calibrated dial).