right click the name of your iPhone in the side bar to the left. Then click 'restore from backup'
If you don't see the side bar, click 'view' in the menu bar, then 'show side bar'
If you're a windows user and don't see the menu bar, click the black/white square button on the top left of your screen and then click 'show menu bar' first.
If you're talking about an iCloud backup, you can only restore from that during configuration.
Go to Settings - general - reset - reset all data and settings. After that you should be able to restore from the iCloud backup, provided the current iPhone is at least the same software version as the phone on which the backup was made.
Message was edited by: jesterwylde