How can I help my dog gain weight?

Easiest way is to probably add more hard food to their diet

You can switch your adult dog back to puppy food which has a much higher fat and calorie content. It really helps an underweight pup gain weight fast. You can also add coconut oil and peanut butter to your dog's food, both of which are high in calories.

Hello

If you would like to help your dog gain weight, you can add an extra meal each day, increase the amount of food you give, be sure the pet food is of good qualify, add Non-fat, no salt chicken broth, beef broth or vegetable broth, canned tuna, non-fat cottage cheese, non-fat plain yogurt, or plain canned pumpkin, add water to diet, gradually change diet to a high calorie diet (puppy food), supplements, less exercise.

If your pup already doesn't eat much nor exercise much, I would talk to your vet about thyroid testing. Usually the thyroid in dogs make them gain weight, but not every time.