The reset option on your iPhone can help you completely reset your iPhone, or do a variety of other tasks to help your phone operate at its optimal level. To get to the reset options on your iPhone, just tap on Settings and then General. After you tap on reset you will see the list of choices to reset all or part of your iPhone.
Reset all Settings on your iPhone will reset all of your preferences to default, but it will not delete any of your data or apps. Reset Network Settings will reset your wireless network to the factory settings without deleting any apps or data. If you want to remove all the custom words and spellings you added to your iPhone’s spellchecker, without deleting apps or data, choose Reset Keyboard Dictionary.
You can also choose Reset Home Screen Layout. This option will reset all of the app arrangements and folders you may have set up. To reset your iPhone’s GPS location permissions to use your location, tap Reset Location Warnings without deleting any apps or data.
Finally, to completely delete the contents of your iPhone including data and apps you may have downloaded, choose Erase All Content and Settings. Make sure you have a backup before you do this if you want to later re-load any data or apps that will be deleted.