If a verification needs changes, the brand's card shows Needs a fix and a Fix my recap button. Open it: the recap shows the reviewer's note. Update what the note asks for, then resubmit.
If you can't provide what's requested — for example, the event is over — click Report a problem on that brand's page in the recap (in the fix sheet: Report a problem instead), choose a reason, explain why in Anything else we should know? (optional), and click Send request.