If you do decide to install the developer preview of iOS 10, however, here's how you do it.
1. Using Safari on the device you want to update to iOS 10, go todeveloper.apple.com, tap on Account and sign in using your Apple ID and password. (If you haven't signed up for a developer account, go and enrol.)
2. Tap on Discover (along the top) and then tap iOS. Tap the blue Download button at the top right.
3. iOS 10 is third on the list. You'll have to tap the chevron on the right to expand its view; the best option now is to tap Download next to the Configuration Profile - the first option. (You can select a larger download for your specific model of device, but they are larger downloads and intended for when you download to a Mac first and then sync across to your device. Stick with the Config Profile.)
4. Now go into Settings, General, Software Update. From this point on installing the iOS 10 developer preview is just like updating iOS normally - just follow the prompts and agree to the various terms & conditions. At one point in the process you'll be prompted to restart the device.
5. Enjoy iOS 10! And if you see any glaring bugs, be nice and let Apple know.
The iPad 2 and iPad 3, the iPad mini 1, the iPhone 4s and the fifth-gen iPod touch all got unlucky this time around. They could run iOS 9, but they can't run iOS 10.
To run iOS 10 you'll need an iPad 4 or later, an iPad mini 2 or later, an iPhone 5 or later, or the sixth-gen iPod touch.
How to update to iOS 9: Which iPhones & iPads can get iOS 9?
We've discussed this question in considerably more detail in a separate article: Can my iPhone or iPad run iOS 9? But we can summarise things here.
Essentially, if your iPhone is an iPhone 4S or later, if your iPad is an iPad 2 or later (or an iPad mini or later) and your iPod touch is fifth-gen, you're safe - Apple has confirmed that all of those devices support iOS 9. Needless to say, the new iPhones - the iPhone 6s, iPhone 6s Plus and the iPhone SE - and the iPad Pro range all come with iOS 9 pre-installed, so you won't need to worry about updating those devices to iOS 9.
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