From the Inbox, tap the message containing the ringtone.
Tap and hold the area that surrounds the attachment box.
Tap the Save icon (located at the top).
By default, the sound file is saved to: Apps > Settings > Storage > Other (Internal shared storage) > (Tap Explore) > Ringtones.
To view the sound file as a ringtone option, it must be moved to: Internal Storage/Ringtones.