• eat more frequently: when you’re underweight, you may feel full faster. Eat five to six smaller meals during the day rather than two or three large meals.
•choose nutrient-rich foods: as part of an overall healthy diet, choose whole-grain breads, pastas and cereals; fruits and vegetables; dairy products; lean protein sources; and nuts and seeds.
•top it off: add extras to your dishes for more calories-such as cheese In casseroles and scrambled eggs, and fat-free dried milk in soups and stews.
• exercise: exercise, especially strength training, can help you gain weight by building up your muscles. Exercise may also stimulate your appetite.
•make every bite count: snack on nuts, peanut butter, cheese, dried fruits and avocado. Have a bedtime snack, such as a peanut butter and jelly sandwich, or a wrap with avocado, sliced vegetables, and lean meat or cheese.
Hope this helps