[Via Webmin Interface]
Log into Webmin
Click on Webmin
Click on Upgrade Webmin
Leave checked "Latest version from www.webmin.com"
Click Upgrade Webmin

[Via root shell]
You can check current version by doing rpm -qa|grep webmin
Navigate to http://www.webmin.com/download.html and copy the link for the latest webmin-*.noarch.rpm file
currently http://prdownloads.sourceforge.net/webadmin/webmin-1.400-1.noarch.rpm
From root shell, run rpm -Uvh URL_HERE
example rpm -Uvh http://prdownloads.sourceforge.net/webadmin/webmin-1.400-1.noarch.rpm