Please can you tell me if you can suggest on how to gain some weight on my dog as she underweight?

Is the dog eating? If not, take it to a vet, it might be sick. Otherwise try smelly things like jack mackerel to stimulate appetite. You'd be surprised, but the cheap store brand foods are the ones that a lot of dogs like. The smellier the better. Also there is a paste available at pet stores called Nutri-cal. It is good for adding calories, but it should not be used as a sole source of nutrition. Combine it with a good dog food. (meaning one that has well balanced nutrition.) Obviously don't combine the paste and the food.

OK thanks