New Font for a Fresh iOS 9
Apple did not drastically change the look of the iPhone and iPad with iOS 9, but there is a new font. The San Francisco font is part of iOS 9 and it delivers a fresh look to the operating system without major changes.
The new font appears in the settings, app names, spotlight, in text fields, Safari and other locations on the iPhone.
As far as we know there is no option to revert to the old font in iOS 9. This new iPhone font matches what is on the Apple Watch and on OS X El Capitan.