If it's an iCloud backup, you have to start by erasing the device. If you have any newer data, such as recent photos, that are not contained in the backup you'll need to save those first.
Then go to Settings>General>Reset, tap Erase All Content and Settings (providing the password if prompted), then go through the setup screens on the device and when given the option, choose Restore from iCloud Backup and follow the prompts.
Be sure your device is connected to wifi and your charger while it is restoring the backup as it will take some time to complete.