Open Preferences → App Passwords (or the Security tab in Classic). Each app password is listed with when it was created and when it was last used.

• Click Revoke next to one to stop it working. The device using it stops syncing within a minute or so.
• Click Revoke all to cancel every app password at once, for example if you think your account has been compromised.

Revoking app passwords never changes your main password or your webmail sign-in. If you think someone knows your main password, change it as well.