Unlocking an iPhone

How to unlock your iPhone for use with a different carrier - Apple Support

https://support.apple.com/en-in/HT201328

Please define "unlock".

If you're referring to the carrier lock, see this document: https://support.apple.com/en-us/HT201328

Depends on how its locked.

Is it locked to a carrier?

Activation locked?

Locked with a passcode?

Some other lock?

Explain what the problem is, and someone may be able to provide some advice.