Select an option from Settings > Sounds & Haptics > Ringtone to set a new default ringtone. You can use a third-party app to create a custom ringtone. Select it as the default in the Ringtone settings ...

Apple has introduced a simple and free way to create and sync your own custom ringtones to the iPhone using GarageBand (i.e., Mac only). The first thing you'll need to do, if you haven't already, is ...