Some iPhones and iPads appear to be having trouble updating to iOS 14 from older versions of iOS. If you receive an “Unable to Install Update” error after downloading iOS 14, it may be worth temporarily disabling your passcode before trying the update again. Make sure to re-enable your passcode once your device successfully updates.
It’s unclear what devices and/or old versions of iOS are affected by this bug, but I confirmed the issue on an iPhone 7 running iOS 10. At the moment, several other users are reporting similar issues on the Apple Developer Forums.
If your device still won’t update with the passcode disabled, check if you have enough free storage space available. It’s also worth taking a look at Apple’s guide on what to do when your iOS device won’t update.