Here are a few ways of cooling down the temperature:
1. Try freezing a coke bottle filled with 2/3 volume of the water. Prepare more bottles to keep recycling the process of cooling down the tank temperature.
2. Put ice cubes in freezer bags. To make more ice cubes, have at least 3 ice trays. Once one tray is used up, refill it with water and place it in the freezer again.
3. If your room has air conditioners, you may try switching it on to cool down the tank. This method is very expensive and unless you can afford to pay the bill, then you'll have to resort to cheaper methods.
4. Set up some mini electric fans on the top of your tank and switch it on. This will fan the surface and helps cool down the tank.
5.Also if it's not possible to have an electric fan blowing on the water, you can have a fan blowing up against the tank glass. But this will not change the temperature a whole lot as the room should be cooler for this to work effectively.
Try monitoring the tank temps when doing these methods.
Another thing: Increased tank temperatures also increase the rate of water evaporation so you'll find yourself doing a water change in bigger volume.
Caution: Be sure to prepare airpumps. Rising temperatures can also reduce the oxygen levels.
Use the 1st and 2nd methods only when you have time to monitor the tank temps. Severe decreasing of temp is stressful to the fish.
The 3rd method is very expensive so resort to the last method if you don't feel like switching the AC and running an electric bill.