1. Login to the admin platform
2. Click on the 
icon and click on "General"
3. Towards the right side of the page, there is a section that says "Custom Ringtone" and click on 
to upload
4. Select the mp3 file that you'll be using as the custom ringtone
5. Once uploaded, hit on 
at the lower right side of the page to save the changes you made
NOTE : This is a global setting, meaning once you do this, it will apply towards your whole platform.