Apple has released iOS 9.0.1. It's a maintenance release that addresses several issues, including (per Apple):
° Fixes an issue where some users could not complete setup assistant after updating;
° Fixes an issue where sometimes alarms and timers could fail to play;
° Fixes an issue in Safari and Photos where pausing video could cause the paused frame to appear distorted;
° Fixes an issue where some users with a custom APN setup via a profile would lose cellular data.
iOS 9 can be downloaded free via iTunes or as an over-the-air update through the Software Update function in iOS's Settings app. To install it, you'll need to be using (at least) an iPhone 4s, iPad 2, or fifth-generation iPod touch.