What should be done to apply a guest OS patch to an existing template?

To apply a guest OS patch to an existing template, converting the template to a virtual machine (VM) is the practical and correct approach. When you have a VM template, it is essentially a snapshot of a pre-configured virtual machine that is used to deploy new VMs. However, to modify its configuration or apply updates and patches to the guest operating system, the template must first be converted back into an active virtual machine.

Once the template is converted to a VM, you can power it on, log in, and apply any patches or updates needed. After the patches are applied successfully, you can re-convert the VM back to a template if desired, which will now include all the updates you made.

This process is essential because templates are not intended for direct modifications or for installation of software patches. They serve as a base for new deployments, meaning they operate under a different model than standard VMs, which are meant to be customized and updated.

Other options like uploading a patch to VMware Update Manager (VUM) or using a snapshot would not directly allow for the application of patches on a template. These methods do not facilitate the needed changes to the guest OS in the manner that converting the template to a VM does.