How to increase maximum battery capacity
If it has fallen below 80% health and it is still covered by warranty or AppleCare, yes. Otherwise, you will need to pay to have the battery replaced.
iPhone Battery Replacement - Official Apple Support
If the battery health level has reached 80% or lower, contact Apple or an authorized dealer to have the battery replaced.
The better replacement should i get free of cost or any cost?
If it has fallen below 80% health and it is still covered by warranty or AppleCare, yes. Otherwise, you will need to pay to have the battery replaced.
How to increase maximum battery capacity
