How do you undo a update .?

If you're referring to a beta iOS version, click here and follow the instructions. They will only work during the beta period.

If you're referring to a released iOS version, you can't.

If you're referring to an application, you'll need to have an older copy of it.

(199852)

If you have updated to a given iOS version then cannot downgrade to a lower iOS version.

Axel F.