I do this all the time go to your contact and select edit scroll down to ring tone touch that and change from there and save, I use zedge for new ring tones.
I had the same problem. Was on the phone with verizon tech support for 2hrs trying everything. Finally realized that some file must have been corrupted when transferring my contacts to the new note 4 Backed up everything on Verizon cloud, did a factory reset. Ringtones now work.