The antonym of "upgrade" is "downgrade", quite broadly used.
What is the antonym of "update"?
Though, I hoped for something like "un-update" because reverting or uninstalling updates are much lengthier.
After having further answers about "Restore", et al, I decided to restore the phrase from my original update.
Yes, I can revert updates (uninstall service packs, hotfixes or restore the operating system to a previous state/image)