Step 1 Launch iTunes and connect your iPhone
Install iTunes 12 on your computer. Connect your device using USB cable. After being detected by iTunes, a device image appears on the upper corner.
Step 2 Recover your iPhone data
Select your iPhone and click the "Restore Backup" button in the "Summary" panel.
In this stage, you are requested to choose a backup from a list and then click the "Restore" icon to recover your data. When it is done, your iPhone will be reset and the files will be back to the device after the resetting.
Note: Other occasions require you to retrieve data from iTunes backup:
1. Transfer information from an old phone to a new one.
2. Be unable to unlock your iPhone for forgetting the password or trying wrong ones several times. (In this situation, you should set your device into Recovery Mode first.)
Not only restore iPhone from backup, iTunes is also a great tool to help iOS users deal with many problems. For iPhone users, I suggest you make sure that iTunes should be always set up on your computer and updated constantly!