Now i am 31 years old. my height is 180 cms. and weight is 63 kgs. how can i increas my weight?

Nothing drastic: If you've been losing weight for unclear reasons, you should be evaluated for that. But if this is your normal, there's not likely to be anything wrong. Your body mass index is within the normal range. If you're not in the best physical shape, you could start doing some resistance training to build muscle mass. No need to get carried away.

Go slow: Eat healthy foods high in calories. Eat 6 meals /day. Work on strength training to build large muscle groups. Work out w heavier weights w fewer repetitions. Sleep at least 8 - 9 hrs/night. Hydrate well w water. Foods to help w weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola, trail mix, dried fruits, protein shakes & bars, lean meats, poultry