The update is 34.1 MB and is being sent out OTA. Since it is being sent out on a staggered basis, there is a chance that you might not receive it for a few days. You can try to coax it out of your phone by going to Settings > General > Software update.
Once you are ready to download and install iOS 9.0.1, make sure that you are connected to a Wi-Fi network. Even though the update is not a large file, you should also make sure that your battery is charged to at least 50%.
Thanks for the tip!
source: Times.news.co.uk