I have every confidence that, with patience, you'll be able to successfully install the iPadOS 26.7 update, or iPadOS 27 upgrade, within a few hours.
Many of us are experiencing extended waiting-periods at the moment due to server loading. While each of us in different regions may experience variable delays, my own iPadOS 26.7 update took about an hour to verify and prepare the install after the package was successfully downloaded. Mrs LP by comparison managed a relatively quick install some hours later. Your mileage will doubtless vary.
If verification of the install package already downloaded continues to be problematic, you can delete it and try to download a fresh copy. To delete the existing download prior to installation, this is achieved from iPad settings:
Settings > General > iPad Storage > Show All >
Scroll through the list of Apps where you will find the update package - the sort order is in diminishing size. When you find the entry for the update, swipe it to the left to expose the Delete/Remove button.
Once removed, you can re-trigger the update process - which will [eventually] download a fresh copy of the update package.