Undoing Claps is not intuitive. The button is hidden within the share menu:
Our redesign of Medium’s Claps…and why they may not have chosen to do it this way.
Jason Li

I’m going to assume this was an intentional decision to make it harder for people to undo applause.

One clap, two clap, three clap, forty?

By clapping more or less, you can signal to us which stories really stand out.