First, I would try a different thermometer to rule out a defect there. Once you know you have a working thermometer, turn off the lights for a few days and keep monitoring the temp. If it rises or doesn't fall, then try lowering the temperature with ice (in a bag if not RO/DI water).
Once you have it just below your ideal temperature, then you should use your heater to keep it at the ideal temp.
While doing all of that, you should confirm that your heater is working. Take it out and put it into a bucket of tap water (cold) and see if makes the temperature rise (keep the water circulating during this test). If you determine your heater is defective, replace.
If you feel you need to address this immediately, then another thing you can do is a water change with a batch of water that is lower in temperature than your tank. If you can do the math (you have to know all the ratios between your batch and your tank), you can figure out what temp your batch needs to be to lower the tank temp by your desired degrees. (Let us know if you can figure this out and we can help with the algebra.) Placing the cold batch in with the warm tank will lower the tank to your desired temperature. You have to be *VERY* careful with this though since a large change in temperature can stress your tank and kill livestock.
I hope this helps.
