When an 'Update' sticker appears on a game on the shelf in 'my collection', what action is needed? Does it mean I have to completely uninstall the game and then re-install it to update it, or is there another (less time-consuming!) alternative? Any help appreciated,thanks.

In most cases you get a separate update file you can download without re-downloading the whole game. But, if you haven't downloaded the game before then you can use the main installer which will be updated to the latest version as well.

Nail. Head. Hit. :D

In most of the cases you have to uninstall the game first (remember to keep your saves and profiles!) and then install the newer version. Sometimes there's also an option to install the newer version on the top of the previous one, without uninstalling.

If you have any doubts, just ask in the thread about the update - Judas is likely to reply in such cases :-)

What stg83 said, though in many cases GOG just updates the installer and it isn't necessary to re-install, just check the game forum to see if anything important has happened.

That depends on how the update was made. If there is only a new installer then yes, you'll have to uninstall the game and install it from scratch with the new installer. But if there is an update patch you can also download that and it will patch your game to the newest version without having to download the full installer or having to reinstall.

However, if the last time you updated was very long ago it might be possible that the update patch does not recognize your installed version. In this case you will have to download the latest installer and reinstall the game.

That's not always true. Quite often, a new patch is added first, and the main installer is only updated several days later. If you need to install the game between those two updates, you'll need both the main installer and the patch.

Yeah, that is why I said in most cases probably should have added "in my experience" as well, Thanks for the info though. :)

Basically what Geralt_of_Rivia said, but also keep in mind that it's quite likely that the update refers to one of the game's extras, in which case you don't need to do anything about your game.

If it's a game-update, it's advisable to backup your saved-games before taking any action, but keep in mind that in some cases old saved-games may not work with an updated version.

If you want to ensure about any update, check the "game-specific" forum. This is my behavior on GOG and Steam. :D