Just take the SIM card out of it and do a master reset on the device and all of your info will be gone. If you are still keeping AT&T service then just put the SIM in another device. If you are cancelling your service then the SIM will automatically be deactivated in that process.
Master reset: Settings>General>Reset>Erase All Content and Settings. This will wipe the device of all content and reset it to factory settings
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