Introduction
How to change the server's hostname from WHM.
Procedure
1. Login to WHM as the root user
2. Navigate to: Home »Networking Setup »Change Hostname
3. Read the instructions on that page to avoid choosing a bad hostame
4. Enter your desired hostname into the box at the bottom of the page
5. Click the blue "Change" button
More details can be found here:
https://docs.cpanel.net/whm/networking-setup/change-hostname/