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 ...

The Retroid Pocket Classic is perfect for an 80s kid who grew up with Game Boys; he just didn't know it existed.

Here's how to change your iPhone's default notification tone.