Updating a game works in the same way that installing a new game does. Updating works on checking MD5 changes in each file based on the origional file on the host, this allows you to only transfer those changes that have been made to the master server and not all files for the game.
Updating works almost in the same way as Installing, however where a new installation will offer all hosts for your service, Updating will only offer hosts that GameCreate has installed for that specific game (shift + click required hosts to update), simply select the host required and click done.
Additionally, GameCreate can optionally shutdown affected servers which use the software thats being updated while it is in progress, and automatically restart them once the update is complete.