This function may not work if the contact is saved on a SIM card.*
Open "Contacts" app : - select a contact, - tap on, - tap on, - tap on "Set ringtone", - select a ringtone from the library, - tap on "OK" to save. or - to choose a personnal ringtone, swipe the screen downward, - tap on , - select a music track, - tap on "OK" to save.