Restoring to an iCloud backup has to be done in the initial setup process, which requires erasing the phone. Go to Settings>General>Reset and tap Erase All Content and Settings. You will then go through the setup screens they did in the store when the phone was new (just follow the on-screen directions). At one point you will be given the option to Restore from iCloud Backup (see image below). Choose this option and be sure you are connected to wifi and your charger as this will take some time to finish.