The most simple and available at home remedy is to place the subject in the fridge. There it is guaranteed not to heat up.
If the refrigerator is not at hand, you can put the object in a bath full of cold water from the tap or put before the conditioner, put on the minimum value.
If there is no air conditioning, no bath, will help ordinary fan. You must wrap the subject matter wet and point it to a jet of air. When water evaporates it cools the surface. Way to cool, say, drinks in bottles. The main thing is to make sure that the fabric is not dry, otherwise the heating process will start again.
Cold small size you can put in a thermos. There it will retain its temperature for several hours.
Outdoors you can use a special frozen packages. They are in any car kit. On each package, usually a printed manual that explains how to use it.
If at hand there is nothing at all cooling, cold can at least tightly wrap any insulator: a fur coat, a cotton blanket, warm jacket. They will create a kind of microclimate, restricting the access of warm air. Keep the cold object will fail, but a couple hours in the heat to win.