If you are using the Medium app installed on an Android phone, the three-dot button at the end of the article contains an option to "Undo claps."

If you are using the Medium app in an iPhone, the "Undo claps" option may instead be inside the "Share" button on the article.

If you are using a desktop/laptop, it appears to me that the "Undo applause" option has vanished. It might be a bug in Medium's system. I am pretty sure that option used to be there.