How do I undo an action?
Open Activity, find the action, and reverse it if it's reversible. The assistant asks before anything sensitive or hard to undo in the first place, so most of what reaches that point is something it prepared and you approved.
What the log can reverse are the mail actions the assistant took on its own — archiving, trashing, filing a message as a reminder, reporting spam, silencing a sender, or unsubscribing. Completing a reminder isn't undone from the log; mark it not-complete from your reminders list instead. Some actions that leave your accounts can't be pulled back, which is exactly why the assistant asks before doing them rather than acting and offering to undo.
See also: What is Activity? · How does it decide to ask versus act?