I get those warning from windows for every new release. I have Norton, and it's never raised any issues with me running the older versions. You can probably ignore it safely, but if you're feeling sketched out, run it through a Virtual Machine and copy & paste the save file and NPC memories over to play. Just don't forget to bring those back with you before closing the VM.
Oh, sorry, I'm not paying attention. Thought it was a download issue.
I can only assume it's a false positive, I've had that happen once before. You should be able to turn your AV off for the download and turn it back on afterwards.
If you don't want to do that, you could also try downloading and running version 1.2.6c and see if you get the same issue.
1.2.6d only has a single niche fix for imported .glbs models not being centered. So unless you're about to make a custom character with a .glb with an offset pivot model, you don't really need to run 1.2.6d.