When you say you want to rename the client, do you mean the entry in the Ghost Console? Or rather the actual client computer?
If you are just talking about the Ghost Console entries what I do, which I'm not sure if it's the proper way, is to highlight the entries I want to refresh and delete them. If they are currently connected to the server they should refresh within a few seconds with the proper name.
Alternatively you can run a refresh configuration I believe to update the client entry in Ghost Console but don't quote me on that as I rarely use that.
If you are wanting to rename the actual client machine you can run a configuration task in the Console. You can manually edit each client machine's entry by going to the properties and editing the default configuration. If you are modifying the name there you can then run a configuration task to apply that to the computer.
Hopefully that clears some things up but if this doesn't make sense I could offer more details.