To change your bakery name: Tap the "Change" button to the right of your bakery's current name. A pop up will appear wherein you may write your new bakery name - note that it costs 10 gems every time you change your name so type carefully!
As for changing your font, that's something that is not done within the app itself but rather your phone settings - the exact details on where to click and tap to do so will depend on what device and OS you're currently running. You'll likely need to google around for the specifics on how to change the font on your device.