If you phone is already set up you have to erase it and go back through the setup screens to access you iCloud backup. 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 you did when the phone was new. When given the option, choose Restore from iCloud Backup (see image below). Be sure you are connected to wifi and your charger as this will take some time to finish.