Shoulder dislocations are extremely rare in cats and dogs. Unlike in human (and I am speaking from experience as I dislocated my shoulder in hockey 4 years ago and popped it back in...not fun!) cats and dogs walk on their front legs and so the musculature is quite different. In 14 years of veterinary experience I have never encountered a dog with a dislocated shoulder. Assuming it is though, you would need the help of a veterinarian. Your Labrador would need to be sedated or anesthetized to allow for correction and then radiographs would be needed to confirm it is put back in the right place. If done incorrectly additional damage to the nerves can occur and serious complications will be the result. I am more concerned however as to why you think the shoulder is dislocated. More often then not there are torn ligaments, a hairline fracture, or other conditions that mimic the signs of a dislocated shoulder. Have a veterinarian examine your dog, I expect radiographs will be recommended and medication will be prescribed.
Good luck.