Official article

Step 1 – Login
Use your credentials to login on the Webex Settings portal
https://settings.webex.com

Step 2 – Select the ellipsis (three dots)

Step 3 – Delete Account
That’s it! you are all set, if you are deleting your free Webex account because you are now subscribed to a service using the same email address then please ask your provider to re-send the onboarding email.