Open the Settings app
Tap Sounds & Haptics
Tap Ringtone
Select the ringtone you want
To use a custom ringtone, add it to your iPhone first
Open GarageBand or iTunes/Finder to create or sync the ringtone
Make sure the ringtone is in .m4r format
Sync or import the ringtone to your iPhone
Return to Settings > Sounds & Haptics > Ringtone
Tap the custom ringtone to set it as your default ringtone