What can endotherms do to reduce their body temperature?

Endotherms have a variety of physiological mechanisms which allow them to reduce their body temperature if it deviates too far away from the norm, for example after strenuous exercise. 

These mechanisms include....

Sweating:
When an endotherm needs to cool down, the glands in their skin secrete sweat. This sweat evaporates from the surface of the skin, and by doing so, removes some of the heat energy away from the skin's surface. 

Vasodilation: 
This is a process in which the muscles relax, meaning the blood vessels' (that supply the skin with blood) dilate( meaning they have an increased diameter). This leads to an increased blood flow near the surface of the skin, which allows more heat to radiate away from the body. 

Flattening of hairs:
The erector pili muscles (in the skin) relax and this therefore results the hairs on the skin to lie flat, this allows air to flow over the skin and therefore allows body temperature to reduce as heat is lost by convection and radiation.

Other ways...
There are a variety of different methods, which aren't physiological processes, which also allow an endotherm to reduce their body temperature. These are mainly behavioural mechanisms such as finding shade, or removing items of clothing.