Go to Clock>Alarm>add or touch the existing alarm to enter the edit interface>Ringtone, then you can touch Custom ringtone to set your custom ringtone.
For Funtouch OS 9.0 and higher version, you can also enter Settings>Sound and vibration>Incoming call/Message/Calendar alert/Notification ringtone>Custom ringtone>select the song which you want to set as a ringtone>Trim>crop the ringtone to edit a ringtone.
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