Perhaps you moved to a new area where your carrier doesn't have great coverage. Or maybe you simply want to lower your monthly bill and found a better deal from another carrier. No matter your reason, it's perfectly legal to unlock your iPhone -- if it's not already unlocked -- and switch to a new carrier.
Your iPhone needs to be fully paid for, however, before you can jump ship. If you are in the middle of an installment plan, you'll need to complete your payments before your carrier will let you leave.
Wait, my iPhone might already be unlocked?
If you paid full price up front for your iPhone, then it's likely unlocked. If you purchased it through AT&T, Verizon or another carrier, then it's probably locked to the carrier's network.
How do I know if it's locked or unlocked?
The best way to check to see if your phone is locked or unlocked is to call your current carrier and ask. You can also check this handy Apple support page to see if "Unlocking" is listed among the features of your carrier.
Alternatively, if you have a SIM card for the carrier to which you want to switch, you could swap in the new SIM card to see if you can make a phone call or send a text. If you can, then your iPhone is unlocked.
What if it's locked?
If your iPhone is paid up, then you'll need to contact your carrier to request an unlock. You may need to wait a day or two for your request to be completed. Once confirmed, you're good to go. Once your new service is up and running, don't forget to cancel your old contract.
OK, so how do I remove the SIM card?
Back up your iPhone, power it off, grab a safety pin and pop it into the little hole next to your SIM tray to eject it. From the iPhone 4 and onward, the SIM tray is located on the right edge of the device.
SIM is out -- now what?
All that's left to do now is to put your new SIM card in the tray and slide it back into your phone.
Yep, one more thing: take your phone number with you. You should be able to transfer your number without issue from one carrier to another, but it's a good idea to check for yourself. In the US, you can check with the major carriers -- AT&T, Sprint, T-Mobile and Verizon -- to see if your number is eligible to port.