Choose General > Reset and tap Reset All Settings.
All your preferences and settings are reset. Information, such as your contacts and calendars, and media, such as your songs and videos, aren’t deleted.
Connect iPhone to your computer or a power adapter. Choose General > Reset and tap “Erase All Content and Settings.”
This resets all settings to their defaults and erases all your information and media:
On iPhone 3GS: by removing the encryption key to the data (which is encrypted using 256-bit AES encryption)
On iPhone and iPhone 3G: by overwriting the data
IMPORTANT: You can’t use iPhone while data is being overwritten. This can take up to two hours or more, depending on the model and storage capacity of your iPhone. (On iPhone 3GS, the removal of the encryption key is immediate.)
Choose General > Reset and tap Reset Network Settings.
When you reset network settings, your list of previously used networks and VPN settings not installed by a configuration profile are removed. Wi-Fi is turned off and then back on, disconnecting you from any network you’re on. The Wi-Fi and “Ask to Join Networks” settings are left turned on.
To remove VPN settings installed by a configuration profile, choose Settings > General > Profile, then select the profile and tap Remove.
Choose General > Reset and tap Reset Keyboard Dictionary.
You add words to the keyboard dictionary by rejecting words iPhone suggests as you type. Tap a word to reject the correction and add the word to the keyboard dictionary. Resetting the keyboard dictionary erases all words you’ve added.
Choose General > Reset and tap Reset Home Screen Layout.
Choose General > Reset and tap Reset Location Warnings.
Location warnings are the requests made by applications (such as Camera, Compass, and Maps) to use Location Services with those applications. iPhone stops presenting the warning for an application the second time you tap OK. Tap Reset Location Warnings to resume the warnings.