Change iPhone XR carrier

If it is locked to a specific carrier, only that carrier can unlock it, if your are eligible.

See this support article:--> How to unlock your iPhone for use with a different carrier - Apple Support

If it is unlocked, you just need to choose a different carrier and use their SIM card.

Regards

Giulio

Contact your carrier to request that they unlock your phone. Start here: https://support.apple.com/en-us/HT201328

Its your carrier that you need to speak to - not Apple