Open the Settings app
Tap Sound and vibration
Tap Phone ringtone or Ringtone
Select a ringtone from the list
Tap Save or OK if prompted
To use a custom sound, tap Add ringtone or choose from Files
Select the audio file you want
Confirm the selection
For contact-specific ringtones, open Contacts
Select a contact
Tap Edit
Tap Ringtone
Choose a ringtone
Save the contact changes