Updating an Older IPad Mini

If you're referring to a first generation model, you can't. Otherwise, use a computer or the Software Update function in Settings.

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I found a solution for my iPad 3 from 2012 that is also stuck running iOS 9.3.5 - the AppStore says Libby is compatible with iOS 9 so I was able to download the latest version, but a change in certificates is causing a problem with the app working.

In Safari, I downloaded an updated certificate at this link - https://letsencrypt.org/certs/isrgrootx1.der

and installed it when prompted. Libby now works on my 11 year old Retina iPad.