They send you a text/email with the time you should go.. you go around that time and they will have two lines.. reservations line and no reservations...
You get in line and wait to be helped by an apple employee. If you do a upgrade swap make sure to tell them not to turn the phone on!
I Also think they hold the phone a certain amount of time.. I think 24 hours. But you need to check the terms for that.
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Where did you read it?