Welcome to Android Central! Are you asking how to change your current ringtone to one of the other preinstalled ones, or how to add new ringtones? To change your current ringtone, go to Settings>Sound>Phone Ringtone and choose a different one.
To add your own ringtone, use a file manager app like ES File Explorer to copy the mp3 or wav file to the Ringtones folder. When you go to select a new ringtone, this file should show up. If it doesn't at first, try waiting for about 10 minutes, or reboot the phone.