First aid is not a substitute for proper veterinary care, but if you have the knowledge and can apply first aid, it will increase the odds of a favourable outcome for your animal. Always seek veterinary medical attention immediately.
Heatstroke: signs may include weakness, panting, vomiting, dizziness, diarrhea, a bright-red tongue and thick saliva. For moderate heatstroke (body temperature 40-41°C), move your pet to a cool place and point a fan at them. Pour rubbing alcohol on the ear flaps and back; if no alcohol, use cool water. No ice or very cold water, as lowering the temperature too quickly can lead to shock.
For severe heatstroke (over 41°C) this is an emergency, get to a vet immediately.