This task is very simple and can be done in two steps:
1.) Login as root and change your server hostname:
hostname newhos.name.tld
Note: Don’t forget to create an A entry for your new hostname, otherwise you will get a popup message during WHM login.
2.) Issue /usr/local/cpanel/cpkeyclt to update your Cpanel License Key, otherwise you will get Invalid License when you login to WHM/Cpanel interface.
A better way to change hostname on cpanel is via the https://server_ip:2087/scripts2/changehostname
This way the change will also be retained after a reboot.