Open the Settings app
Tap Sound and vibration or Sounds and haptics
Tap Ringtone
Select the song you want
If the song is not listed, add it to your music library or ringtone folder first
On iPhone, use GarageBand or iTunes to create and set a custom ringtone
On Android, use a ringtone app or file manager to move the song to the Ringtones folder
Confirm and save the selection
Set the song as the default ringtone for calls
Set the song as a contact-specific ringtone if needed