Welcome to Android Central! If you connect the phone via USB, the phone should appear in Windows Explorer (assuming you have a Windows PC). You can then drag and drop any music files from your computer to the phone. Place the files in the Ringtones folder. Once you disconnect the phone and go to Settings>Sound>Ringtones, you should see the new music files listed. If not, you may have to reboot the phone first.
You can also upload the music files to a cloud service like Google Drive, then download them to your phone using the Google Drive app. Once on your phone, you can move the file to the Ringtones folder using any file manager app (like ES File Explorer).