How can I unlock my phone to use a different provider

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

Depends on the carrier and its unlock policies. Verizon's policies are that they automatically unlock phones after 60 days of being activated, as long as the account in in good standing. Once unlocked they can't re-lock it to the carrier. There are other policies that they might have over deployed members of the US military. Other carriers require that the unlock request be done by phone or via a website.

You contact the carrier it is locked to.

Apple doesn't unlock phones.