Error message when trying to install iOS 14 update

These support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

Be aware that your iPad may be too far behind the current version to be updated directly from iPad settings. As such, you will likely need to use a computer to perform the update. You will need access to either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

What software update are you currently on?

If you are too far behind in updates the iPad will not be able to process the update. If that is the case-

On a Mac or PC (with iTunes for Windows) connect your device. It should show up in the sidebar of Finder/iTunes. If it is not already trusted, click Trust and a prompt will appear on your iPad. Then click Check for Updates and then update. Do not disconnect your iPad during the update.

If you don't have one of those computers then borrow one from a friend or go to a Genius Bar.

Good luck. Have a nice day.