Those running an older version of Webmin should upgrade..
Here's a quick and easy way for RPM users.
This is for Redhat Linux Distros.
First check the webmin version. If you are already using the latest version, then there is no need to continue this tutorial.
rpm -aq | grep webmin
Create a folder to download the webmin rpm.
[root@server root]# mkdir /root/setup
Go into the folder.
[root@server root]# cd /root/setup
Download the rpm.
[root@server setup]# wget http://prdownloads.sourceforge.net/webadmin/webmin-1.310-1.noarch.rpm
Install the rpm.
[root@server setup]# rpm -Uvh webmin-1.310-1.noarch.rpm
After it installs, please restart webmin.
[root@server setup]# service webmin restart
You should be good to go.

EXTRA Information
To change your root password, type the following and you will be asked for a new password.
[root@server setup]# passwd root
Samba users, please check the following file and delete it. You will be asked to verify the deletion. Type 'y' for yes and press enter.
[root@server setup]# rm /etc/webmin/samba/swat
rm: remove regular empty file `/etc/webmin/samba/swat'? y