Help me with my ticket situation

have a flight from Delhi to ktm in Nepal airlines ,
I want to change the date from 11 June to 13june, it’s and economy class but because of the 4 day public holiday I’m about able to. I want to change on Monday as I’ll still have more than a week left before my flight. But the question is will be able to change it and how much penalty will I have to pay. It’s a gamble cause I’m going to book air bnb tdy for longer time than my flight. Do you have any idea about the airfare that I’ll have to pay more?