Yes you can give your dog Pepto for an upset stomach.
The recommended and safe dosage of Pepto for a dogs upset stomach is 1 teaspoon for every 10 lbs of the dogs weight.
So if the dog weighs 10 lbs you'd give the dog 1 teaspoon of Pepto and for a 20 lb dog you'd give them 2 teaspoons of Pepto.
Just increase the dosage for the weight of the dog.
For a 40 lb dog the dosage of Pepto would be around 4 teaspoons of Pepto and so on and so forth.
I give my dog Pepto for his upset stomach and it helps him.
You may have to add some Pepto to the dogs food such as canned food to get them to take it.