The Hostname is changed in the configuration mode.

LYSH@MyHostName# configure
configure# hostname set -h
usage: hostname set [-h] name

positional arguments:
  name        hostname

optional arguments:
  -h, --help  show this help message and exit

Example:

LYSH@MyHostName# configure
configure# hostname set MyHostName
configure# exit
LYSH@MyHostName# show system		// The hostname is shown in the "System name"