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 ...

Open the Clock app and go to the "Alarms" tab, then select the alarm you want to edit. Tap the "Sound" option and scroll down to choose a new preinstalled sound or tap "Pick a Song" to choose a song ...

Next up is the action button, up on the left-hand side as you look at the iPhone, which has replaced the old ring / silent ...