Hey I'm fairly new to this, but I had a similar issue
Does your tank get direct sunlight? Mine did at the start, and simply moving it into a place that never gets direct sunlight allowed for a stable temperature, at a temperature that I control.
Without sounding condescending, do you have a heater on? Is it too high? If it's set at 75 but the water is 86+ then either it's broken, or something else is causing the rise.
I've seen post about people adding ice cubes - you could try this, but just remember once the ice has melted on the top then the water has to circulate... Don't just put in a whole load and see no instant effect, so you put in a whole load more.
What temperature is your tap water? You could do some water changes.
What is your room temperature? Use the thermometer that you've got in your tank... If its saying that your room temp is 86 degrees, and if this seems wrong, then it could just be your thermometer. Also remember if the room got really hot and suddenly got cool, you wouldn't notice an instant change in the water temperature.
Just remember if you have fish in there then really ease it down

chances are they aren't enjoying the temperature right now, but if you suddenly drop the temperature then the shock wouldn't be great (but then this is just what my head says. I'm very very very new to this too).
What fish do you have? This could be helpful for the experienced guys with the actual knowledge...