How do I revoke a link I already sent?

Revoking is one click, and it takes effect immediately.

Open the Share or Delivery panel and find the link.
Click the three dots at the top right of the card, then Revoke link.

The next person to open that URL is told the link is closed, including anyone who already had the tab open, because the file is served through the same check. Revoking is you closing a link on purpose; an expiry date is you deciding in advance when it closes. Neither is a link going dead because a plan ran out, which is what happens to a file-sharing link.

Revoked links stay in the list, dimmed and without a copy button. That is deliberate: the useful thing after a leak is being able to say which link was live and when it stopped, and a list that quietly forgets cannot tell you that.

Revoking cannot be undone. If you close one by mistake, make a new link and send that; the old URL stays closed for good. A revoked link that could come back would make the record above worthless, because "this link was live until Tuesday" would stop being a fact about the link.