@hughug There is no default program for opening package files in windows normally. There are programs you can download and install that can open them, but there's no normal default.
Normally if you download a package file, you just put it straight into your mods folder. No need to open it or do anything with it other than move it.
There's quite a few ways to undo to remove the association, but none of them are particularly straightforward. I'd just edit the registry, but that's not something I'm comfortable explaining how to do to someone who I don't know exactly how tech savvy they are.
This page has several different methods you can use, step-by-step...maybe scroll through it and find one that you think would not be too difficult for you (Option #5 at the bottom is the simplest, if you are willing to download and use a 3rd party app):
https://troubleshooter.xyz/wiki/how-to-remove-file-type-associations-in-windows-10/