Eat more nutrient-dense, high-calorie foods like rice, roti, milk, curd, almonds, bananas, eggs, peanut butter, potatoes, and healthy snacks in between meals if you want to gain weight in a healthy way. Make sure you consume adequate protein from foods like dal, chicken, fish, paneer, or soy products. You can also attempt modest strength training to gain muscle rather than just fat. Reduce stress, get enough sleep, and don't miss meals. See a doctor to look for underlying health issues if you are losing weight on your own, have low appetite, diarrhoea, fever, or a chronic condition.