The mother will either leave it, eat it, or feed it too the litter.
What Does Mother Dog Do When Puppy Dies At Birth?
It is hard to say all dogs are different. Some may be upset and I have herd of some eating the puppy. I think they do that so it won't bring any predators up and attack the other puppies.
I'm not 100% sure but when a puppy dies at birth the mom it eats it so other predators don't come over and eat the surviving pups