Restoring to an iCloud backup has to be done in the initial device setup process. If you have any newer data on your phone now that is not in the backup that you want to save you'll need to do that first. When you're done, go to Settings>General>Reset and tap Erase All Content and Settings. Then go through the setup screens on your phone and when given the option, choose Restore from iCloud Backup. Be sure it is connected to wifi and your charger as this will take some time to finish.