It's easy. From a command line, and with root privileges, run one of the following commands:

/etc/init.d/webmin restart

or

service webmin restart

 

After running one of those commands, you'll see the following messages.

Stopping Webmin server in /usr/share/webmin
Starting Webmin server in /usr/share/webmin

Now the Webmin server should be running after a fresh restart.