Restoring to an iCloud backup has to be done in the initial setup process. If you have any data on your phone now that is not in the backup that you want to save you'll need to do that first. Then go to Settings>General>Reset and tap Erase All Content and Settings. You will then go through the setup screens as as was done 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. Be sure you are connected to wifi and your charger as this will take some time to finish.