You're going to have to download the game again. Old patches are no longer kept available.

Thanks for the quick reply :-)

I wonder if the updates fixed anything critical, or just added DLC?

Hi arongog.

GOG.com backup installers get updated from time to time, so links to old patches are rendered obsolete and removed, since they are directly integrated in the updated main installers.
So, unfortunately you have two (2) options.
As mentioned by Hickory, you can download the latest backup installers for the entire game.
Or choose to update the game using GOG Galaxy. After installing GOG Galaxy, import the folder where "The Witcher 3" is currently installed on your hard drive, and let GOG Galaxy update it. I have to warn you though that the volume of the data to be downloaded is still huge, so you might want to re-download the entire game either way.
Patch notes can be found in detail here.
https://witcher.fandom.com/wiki/Patch
Patch 1.32 adds support for the Chinese language.

Cheers.

Thanks for the advice :-)

My internet is limited now, so I guess I'll have to consider whether the update is worth a few months of data… Thanks again for the additional info, I just spotted you edited the post.

It seems like I might need to re-download the files as there seems to be quite a few bugs that have been fixed by the updates.

Hi again arongog.

If I may suggest downloading the game from your local net-café, storing it in an external drive in case of any future decision of installing/uninstalling. It certainly saves up a lot of money and bandwidth, if this option is available to you.

Cheers.

Thanks again, that's a pretty good idea :-)