Open the Phone app and tap on the Menu icon (the three dots on the right-hand side of the bottom row of the screen). Select Settings, then select ‘Call settings’. On the following screen, tap ‘Phone ringtone’. You will then be presented with a list of all the available ringtones on your phone.
Each ringtone has a radio button and tapping on a ringtone will play a preview. To actually set a ringtone, check its radio button and then tap OK. Third-party apps are able to add to the list, which can be misleading if the tune is inappropriately named (‘Default Ringtone’ etc).