If you want to change the default ring tone, go to settings/sounds and vibration/ ringtone. If you want to use one you have downloaded, when you select ringtone it will pop up a list. Scroll down to the bottom and there should be a + sign that says Add from Phone. Click that and go to the download folder and choose the tone.
You can cheat and install Zedge from the play store to find and set your ring tone, but I'd recommend deleting the app after that lol.
You can change contact ring tones by going to the contact and select edit, then More at the bottom. There it will allow to you set custom ring and notification tones for the specific contact.
TL/DR sorry.