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You should be eating healthy nutritious balanced diet which should include fresh fruits and Green Leafy Veggies. Avoid Junk and Drink Plenty of Water. Eat Protein rich foods like Milk, Eggs, Chicken, Fish twice a week this will improve your Muscle mass. Exercise regularly like cardio, crunches, skipping. This will not only increase your External Appearance but also your Appetite helping you eat more., thence gaining weight.
and sleep adequately. Follow this routine for a Month, you should see better changes.