Sure, here are the simplified steps for a customer to change their domain name's WHOIS information from within their WHMCS account:
1. Log in to your account centre using your credentials.
2. Navigate to the "Domains" section in the main menu and select "My Domains"
3. Find the domain name whose WHOIS information you want to update and click on the settings dropdown
4. In the settings dropdown click on "Update Contact Information"
5. Enter the new WHOIS information for the domain name, including the registrant name, organisation, and contact details.
6. Click the "Save Changes" button.
That's it! The domain name's WHOIS information will be updated with the new information you provided. Keep in mind that it may take up to 24 hours for the updated information to be reflected in the WHOIS database.