For the iPhone 6, the upgrade is 1GB in size, so it is recommended that you download using a WiFi conection.
The iOS 9 update is supported on the iPhone 4S and above, while for the iPad, it works with the iPad 2 and above. It is also supported for the iPod Touch 5th and 6th generation. For the iPad Air 2, you do get an extra split window feature which lets you run 2 apps side by side.
To get iOS 9, you can check under Settings > General > Software Updates.
If you’re having problems with the software upgrade, you probably might want to wait for a few hours due to heavy congestion. Alternatively you can try updating through iTunes which requires a direct cable connection to a computer. The 2nd remedy is also recommended for those who are still stuck with the iOS 9 Beta version.
To find out what’s new, you can check out our 9 new things to look forward to with iOS 9 post.