Pour water on their body but not their head, focusing on their stomach, neck, and thighs. “Submerge their body in cool water if available (such as a paddling pool or stream, as long as the water temperature is cooler than the dog),” the RSPCA added, but don’t do this with an unconscious, sick, or older dog.
Article source: https://www.huffingtonpost.co.uk/entry/signs-heatstroke-dog-mouth_uk_6a159e2be4b062ca52d52394