Custom ringtones on an iPhone 5 without iTunes?

I have an iPhone 5, but my computer is so old that I can't run the latest version of iTunes on it. This means I can't sync my iPhone to my computer at all.

I have a sound clip on my computer I want to make into a ringtone. It's already an m4r; I just need a way to transfer it to my phone without connecting my phone to my computer. Is there any way to do this without jailbreaking?

Thanks!