...from my iPad.

how do I undo it?

Not sure on your iPad. On a regular pc I would back up one page (to where it was still unread) and open each thread in a separate tab, thus keeping the original unread forum intact. Other than that, I think Brett would have to go into the database and change some stuff which would be a PITA.

Well, I clicked it as read accidentally on my iPad, but it carried over to my desktop.

Not sure. Maybe ask a mod.

Try clearing cookies and see if that works.