Open the Settings app
Tap Sounds & Haptics
Tap Ringtone
Select a built-in ringtone
To buy more, open the iTunes Store app
Search for ringtones
Purchase and download the ringtone
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select the downloaded ringtone
To use a custom ringtone, create it in GarageBand
Export the GarageBand file as a ringtone
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Choose the custom ringtone
To assign a ringtone to a contact, open Contacts
Select a contact
Tap Edit
Tap Ringtone
Choose a ringtone
Tap Done