1. Log into your cpanel account.
2. In the "Domains" section, click on "Subdomains" Icon.
3. Under the "Modify a Subdomain" list you can see a list of subdomains.
Click the "Remove" option under the "Actions" tab.
5. You will receive a warning that you are removing a "subdomain.example.com". You will need to "Delete Subdomain" to remove your subdomain permanently.