The airline provides too many facilities that are helpful for you. If you skip out the free change window then you have to pay a certain amount. Read below some points about Indigo airline change booking charges- If passengers are making a change before 4 days from the departure then they have to pay 2500INR to make any kind of changes. If you are making changes in 0to 3 days from the departure then you have to pay of 4000.

https://airlinespolicy.com/flight-change-policy/indigo-airlines-flight-change-policy/