How do i update iPad software without plugging into computer or iTunes?

"You might be unable to update your iPhone, iPad, or iPod touch wirelessly — or over the air — for one of these reasons", with more detail in this link. --> If your iPhone or iPad won't update - Apple Support

" Not enough available storage space on your device

Downloading the update takes a long time

Unable to reach the update server

The update doesn't complete"

I also know that we have an iPhone 5s that has almost nothing on it and more than 10 GB free. Still, I can't update it without a computer. It may be that as is the case with your iOS 12 that the jump to the version it will want to install is too great for it to do it on its own.