Buy or download a ringtone file in M4R format
Keep the ringtone under 40 seconds
Open Finder on Mac or iTunes on Windows
Connect your iPhone to your computer with a USB cable
Drag the M4R file into your iPhone in Finder or iTunes
Sync your iPhone
On iPhone, open Settings
Tap Sounds & Haptics
Tap Ringtone
Select the new ringtone
If using GarageBand, import the audio file into GarageBand
Trim the audio to 30 seconds or less
Export it as a ringtone from GarageBand
Set the exported ringtone in Settings