...

you would have to contact gog
go to your game library
then No Man's Sky
click on more
then go down to support

and then down to the bottom of the page where it says CONTACT US and ask for a previous version

as far as i know you would have to uninstall the current version and then re install the old one whilst leaving your saves

personally i had saved the patches from way back and had updated till frontiers
that's when the crashes happened
i'e go to land near a building craaashh then it was get out of my ship craaaashh
that was fixed by patch 2.6.1 so i'm just going to stick with 2.6.1 for now

On mayor updates, I usually rename the old installation like "M:\No Man's Sky_Prism" and make a copy of "C:\Users\%USER%\AppData\Roaming\HelloGames\NMS" and rename it like "NMS_Prism". Then I download the offline full installer and make a fresh installation to "M:\No Man's Sky". After that, I copy my settings from "M:\No Man's Sky_Prism\Binaries\SETTINGS" to "M:\No Man's Sky\Binaries\SETTINGS" to have my keyboard layout and stuff like I want it.
If things go sour, I rename the new installation like "_Frontier" and remove the append from the old installation name and everything is fine again. If then new patches are up, I just revert the renaming, install the patch to the new installation, test it and can, if necessary, revert back to wait for another patch.
Modern problems require .. well solutions we did already in 1985.. Such a shame no one come up with a GIT-like branching system to have different versions side-by-side.

Sorry for the bump, but just wanted to mention there are links and information on my website No Man's Sky Retro (nomansskyretro (dot) com) about how to install and play old versions of No Man's Sky. For the GOG version you can install the original release, foundation, path finder, and atlas rises, but for Steam you can theoretically install any version.

GOG support don't provide old installers any more as far as I know.