kind of too warm. I don't like 84*. What temp in your room? If your room 76-80* i would not use that heater. He would be fine in your room temp. Then you can buy adjustable heater.
2.65 make 1-50% and 1-100% water changes a week.
When you change his water you need to make sure new water is approx +-2-4* the same as his water. You can let water sit overnight and then just mix room temp with hotter water that you can prepare just before you change him.
Take time to acclimate your betta. Have him in his changing cup. Add about 15% of the new water to his cup. Let him used to if for about 5-10 min then do it a few more times. This way he get used to new temp/chemistry.
Make sure you cover the cup if you leave him without seeing him. Bettas can jumped out ...even in his tank always leave space between the top of the tank and water level.
Is your thermometer like this? Then there is the long line to the left from 80* which will be 85* then long line to the right which will be 90* and so on. And i guess the lines between short lines it about 2*.