How can i gain weigh? i am 1m70 and 54kg

Lean is healthy: First let me say that lean people are healthy people. So you might want to consider healthy weight gain instead of unhealthy weight gain. Unhealthy weight is deposited by consuming empty calories. Junk food is a good source. Fries, burgers candy, ice cream, etc. healthy weight gain can be achieved with increasing your muscle mass with weight bearing exercises and high calorie food.