Please refer to this short guide put together by WHMCS support team for information on how to uninstall WHMCS from your server:
https://docs.whmcs.com/Uninstall_WHMCS
The uninstallation process is relatively straightforward and should be easy to carry out by anyone.
Note: If you have not purchased your WHMCS licensed through cPanel, we will not be able to assist with the uninstallation process.