The fastest way to lower your dog's temperature is to move them out of direct sunlight and provide cool drinking water. You can also use damp cloths or dog cooling vests to help them cool down and return to a more normal body temperature. That said, it's important not to rush the dog's body temperature; try to cool your dog slowly over time.
If the dog is panting heavily or showing other signs of potential heat stroke, immediately reach out to your vet for recommendations. Overheating can be deadly for dogs, so it's a condition that should be taken seriously and addressed with urgency.