Restore from a backup of the iphone (in either itunes or icloud).
If you do have a backup somewhere...
Go to Settings>General>Reset and tap Erase All Content and Settings. This will erase your device. Then you will go through the setup screens again as you did when your device was new, and when given the option, select Restore from iCloud Backup.
This link gives another overview of backup and restore:
HOWEVER... you may run into a snag, namely when the newly restored data is downloaded, an automatic icloud sync may occur that will wipe out any Apple data (contacts, calendars, etc) that was restored. If you have such an issue, read the following that describes restoring contacts. The info will also pertain to other Apple data.