How can I reset my iPad stuck on software update?

I’m not sure if it will let you whilst updating, but give it a shot: do this really quickly. Volume up, straight to volume down and then hold the power button until you see the apple logo. If this doesn’t work, go to iTunes on your windows device/ Mac and press update. If it works successfully, finish. If not, press restore. WARNING: restore will remove all your apps and data. If your backed up with your Apple account, then it shouldn’t matter. That’s all I can say. Sometimes, just power it off and leave it for a few days; that’s what I did when my iPad got water damage. A few days later, powered on and ready to go; after it not restoring and iTunes saying hardware error.

This is a known issue - encountered when setting-up a new (unused) iPad and updating to iPadOS 18.x. You will need to interrupt the stalled process with a forced-restart:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

After restarting the iPad, allow the new iPad to complete initial setup and software update without attempting to transfer data from the old iPad - setting-up using your AppleID (now known as your Apple Account).

When the update process has completed, reset the iPad - and THEN perform the transfer:

Settings > General > Transfer or Reset iPad > Erase All Content & Settings

Reboot it and don't turn it on for 10-15 minutes. It worked for me.