To change the default ringtone for all incoming calls tap on Settings-Sounds-Ringtone then choose the ringtone you like. You can also set custom ringtones for your contacts as well. Just tap Contacts, then tap on the contact you want to assign a ringtone to, then tap Edit. Then just scroll down until you see Ringtone, once you tap that it will then allow you to choose the custom ringtone for that contact.