No problem. Although the tone Update is listed under the Alert Tones, it still can be used as a ringtone (I can on my iPhone 5S, iOS 8.1.2). So, please again, go to Settings > Sounds > Ringtone > scroll down till the end of the list (I mean really to the end of the list, you will see ALERT TONES in this very same list, but don't just stop and scroll down till the end) and tap on Classic and scroll down again till the end of the list. Tap Update and you must be done!
If that's not the same on iPhone 6, iOS 8.1.2, then I am sorry but you will have to buy the said tone (don't just download by googling it since it's not legal) and sync it via iTunes.