sim unlock

If it is locked on to a USA carrier then you cannot unlock it in India.

Carrier Lock:- Though your iPhone’s original Carrier is not obligatory to unlock your iPhone, but you may know the process here --> How to unlock your iPhone for use with a different carrier ...

The SIM lock can only be released by the carrier that placed the lock. Contact that carrier in the US and ask them the procedure for having the lock removed. Good luck.