Apple How do I transfer a ringtone to an iPhone 6s?

The steps change from time to time but this is a recent enough tutorial that maybe it still works: https://www.howtogeek.com/248489/how-to-add-custom-ringtones-to-your-iphone/
Good luck!

 

iTunes is an one-way tool and has copyright protection, so that it has no ability to export ringtones from the device to the computer directly. That's why you need an iPhone Ringtone Transfer program to act the best assistant here. With the MobiKin Assistant for iOS, you can transfer or copy ringtones from iPhone 6S to PC selectively or in a batch. In addition to the ringtones, other files like music, movie, photos, playlist, etc. can also be transferred to your personal computer with the help of iPhone Ringtone Transfer program.

 

Here is the guide to solve your problem check the given link

 

Go to the File menu
select Add File to library/Add Folder to Library,
then you can choose the ringtones on your computer and transfer them to iTunes.
Click on your iPhone, click the “Tones” to open the ringtones tab.
Check the box for “Sync Tones” and then check the boxes for the ringtones you want to sync.

 

I used to download and transfer ringtone to my smartphone but have decided to just use the sounds that is available on the device since it will only take some space.