If you're not going to change the branch, normal updates will keep you with latests things. You'll notice when a new version is about to go out, the artwork will change showing the new version. That's 4, 4.1, 4.2, ...
If you're going to change branch, equo hop <branch> (current branch is 4, next one will be 5 afaik) and then update as usual is all you need.
You can't update from a sabayon live-dvd so no need to download it unless you want a clean install for any reason.
Former Sabayon staff (retired).
For any personal questions or whatever, contact me trough my G+ profile