From the Inbox, tap the message containing the ringtone.
Tap and hold the area that surrounds the attachment box.
Tap the Save icon (at the top).
By default, the sound file is saved to: Internal Storage/Download.
To view the sound file as a ringtone option, it must be moved to: Internal Storage/Ringtones.
You can use File Manager (Apps icon > File Manager) to move files.