How do I unlock a carrier SIM-locked device?

Ask the wireless carrier involved if they'll unlock it, or visit their website. Apple won't do this unless they accidentally replaced an unlocked unit with a locked one, and nobody else can.

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Only the carrier to whom the phone is locked can release the lock, if they choose to do so.

if you are not their customer, they might refuse to do so. If you are their customer, they might require that you pay off any contract before they will unlock the phone.