for MAC OSX users
yop,
if you have a mac
you can use iphone browser software like "phone disk",
also if you are unable to delete or overwrite the ringtone on the iphone,
make sure you don't only rename the file extension but also change the "open with" (from iTunes to Quicktime) on the file information of your custom ringtone, and voila you're done https://forums.macrumors.com/images/smilies/biggrin.gif
hope it helps
yop,
if you have a mac

also if you are unable to delete or overwrite the ringtone on the iphone,
make sure you don't only rename the file extension but also change the "open with" (from iTunes to Quicktime) on the file information of your custom ringtone, and voila you're done https://forums.macrumors.com/images/smilies/biggrin.gif
hope it helps