Apple launched iOS 9 last week, and although the upgrade process went relatively smoothly for the majority of people, some bugs have cropped up that Apple has decided to quickly fix through the launch of iOS 9's first update.
iOS 9.0.1 addresses an issue where some people were stuck on the "slide to upgrade" screen, preventing people from upgrading their devices to iOS 9 at all. Another issue, and one that seems to crop up every few iOS updates, resulted in timers and alarms failing to play; again this issue has been fixed in iOS 9.0.1.
In general iOS 9's launch was smoother than iOS 8's launch, but for those people that did run into issues, Apple has uploaded a support document that details how to properly install iOS 9.0.1.
iOS 9 has been Apple's biggest software update yet, with more than 50% of users upgrading to the new operating system within five days of its launch. According to Apple's Phil Schiller, iOS 9 is "on pace to be downloaded by more users than any other software release in Apple’s history."
If you are an iOS user it's probably a good idea to upgrade to iOS 9.0.1 right away to prevent any of the aforementioned issues, especially the alarm issue, from affecting you. The over the air update should already be available across the globe.