Locked sim how can i unlock it?

If your actual SIM is locked, you’ll need to correctly enter the 4-8 digit PIN to unlock it.

Use a SIM PIN for your iPhone or iPad - Apple Support

If you mean that your phone is locked to another carrier and your SIM will not work, you’ll need to contact the “other / locking carrier.

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

If you're talking about a locked SIM PIN, contact your carrier and ask them fro the PUK code.

If you're talking about a locked iPhone to a different carrier, only the carrier whom that device is locked to can help you, if you're eligible. See also this support article:--> How to unlock your iPhone for use with a different carrier - Apple Support

Regards

Giulio