Sorry to hear that. You'll probably have to try more advanced methods then. I'm not familiar with them, but Scifan has suggested one method. Here are some links that may help. I found them on Apple's support pages.
To update your iPhone or iPad, you need a compatible device, an internet connection and enough storage. It also helps to keep your device connected to a power source.
support.apple.com
Learn how to update your iPhone, iPad, or iPod touch to the latest version of iOS or iPadOS using your computer.
support.apple.com
Learn how to update your iPhone, iPad, or iPod touch to the latest version of iOS or iPadOS using your computer.
support.apple.com
Also check to see how much local storage is left on your iPad. I don't know how much free storage an update needs, but it would be best to have a t leas a few GB available. If you're down to less than 1 GB it could be an issue.