You sign up for a new plan and purchase a new device (often on an installment plan).
The good news? Many major carriers are so eager for your business that they will literally . Here is how you can make the switch without breaking the bank. How the "Contract Buyout" Works
To get the full buyout value, your trade-in phone usually needs to be in good working condition (no cracked screens or water damage).
Breaking Up is Easy: How to Switch Carriers and Get Your Contract Paid Off