To change the "Powered By" footer on your WHMCS landing page, follow these steps.

For this change, you will be logging in directly to the Reseller Dashboard, not your WHMCS instance.
Navigate to the "Settings" Tab.
Locate "Account Details" in the section labeled "Social Links (optional)". The URL entered in the "Website Link" field will reflect on your WHMCS instance.
Select "Save" to save your changes.