Open the Settings app
Tap Sounds & Haptics
Tap the sound you want to change
Select a tone from the list
To get more tones, open the iTunes Store app
Tap More
Tap Tones
Browse and buy tones
Download the tone to your iPhone
Return to Settings
Tap Sounds & Haptics
Select the downloaded tone
To use a custom tone, sync it from a computer with Finder or iTunes
To set a tone for a specific contact, open Contacts
Select the contact
Tap Edit
Tap Ringtone or Text Tone
Choose a tone and tap Done