Virtualmin is a fantastic piece of software which allows the not-so-techy run a full hosting server nicely, however I’ve come accross a bug that seems to happen mostly in Centos 5 when trying to upgrade and you happened to have used the rpm to install
This is how you do it.
Login to Virtualmin. Click on System Settings . Click on “Upgrade to Virtualmin Pro” and enter your serial number etc.
Now if you get the following error
Failed to upgrade to Virtualmin Pro : No Virtualmin GPL repository was found in /etc/yum.repos.d/virtualmin.repo
Put this into /etc/yum.repos.d/virtualmin.repo
[virtualmin]
name=Red Hat Enterprise $releasever - $basearch - Virtualmin
baseurl=http://software.virtualmin.com/gpl/rhel/$releasever/$basearch/
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-virtualmin
gpgcheck=1
[virtualmin-universal]
name=Virtualmin Distribution Neutral
baseurl=http://software.virtualmin.com/gpl/universal/
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-virtualmin
gpgcheck=1
Then try again. It should all go well now for you and be able to update.