Download the update but does not want to install

Are you certain that an update has been downloaded by your iPad - but not installed?

If an uninstalled updated is present on your iPad, you can find it here:

Settings > General > iPad Storage - scroll down the list of Apps until you find an Apple Update

If you are unable to find an update package, then it hasn’t been downloaded. If you do find an update, it can be deleted; just “tap” the update, then select Delete Update. After deleting a failed update, you can repeat the update attempt.

The tags appended to your original post suggests that you have an first generation iPad Air. If so, you should be able to update from iOS11.x directly to iOS 12.4.7 using the OTA Update method from Settings. If this is not successful, you can also update your iPad from a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS).

Update software on iOS and iPadOS devices in iTunes on PC - Apple Support

If your iPad Air is still running iOS 11, it is too far behind in iOS upgrades/updates.

If you have access to a full computer, like a Mac or Windows PC, try either using the Finder on a Mac that has the newest macOS Catalina installed OR with an older Mac with an older Mac OS, download and install the latest iTunes version ( or latest iTunes for Windows PC ), make sure the Mac or Windows PC is connected to the Internet, connect your iPad by the USB charge cable to the computer and let the iTunes application upgrade/update that 2013 iPad Air model, using the iTunes Software Update feature, to its current, maximum iOS of iOS 12.4.7 for a 1st gen, 2013 iPad Air, OR iPadOS 13.5.1 for a 2014, 2nd gen iPad Air model.

Best of Luck to You!