GEt some ice in a sealed bag or bottle and float it in the tank, as it melts it will cool the water around it by convection and not ad unreated water to your tank. HTH
I don't have guppies but I keep my tanks at 26.5c/80f
EDIT
found this on the net
An ideal temperature for adult guppies is 22c/72f to 24.5c/76 F. Fry are often raised in warmer water for the first 2 or 3 months (25.5c/78f - 26.5c/80 F).
81 is too warm for adult guppies, but the fry should be kept in warmer conditions(i keep my fry at 78-79F). I keep my guppy adult tanks at 76F. Another way to help cool it down, is leaving the tank lid up while have a window open.
my tank is always at 75 F and all my guppys love it ...
another way to get it cold down is by like someone else said leave the tank lid up and put a Fan near buy to move some air around you can also do a small water change .... if you have a aquarium themometer take that , remove some of the tank water and get some cooler new water using the themometer to make sure the new water is right at 70F before putting it in the tank should help alittle