Step One
Launch iTunes from your dock.
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Step Two
Connect your iPhone to the computer via the USB cable and select it from the iTunes sidebar or at the top right of the screen if you're using the new layout.

Step Three
Make sure you are on the Summary tab and click the large Back Up Now button.
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Step Four
Notice the progress bar will tell you the status of your backup.
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Step Five
Once complete the date and time of your last backup will be updated.
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Step Six
To see all the device backups on your system select Preferences from the iTunes menu.
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Step Seven
You can find the list of backups in the Device tab.
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