I have a theme with partials like: layouts/some-folder/. I would like to reset this to use the default layout, how do I do this easily?
What do you mean?
There is no such as default layout in hugo.
You have Themes (multiple) /themes/ or you have one in /layouts/.
Remove all in /layouts/ folder and download desired theme in /themes/ folder. Select theme in config and use it.
Everything in your /layouts/ folder overrides things in /themes/mytheme/layouts/ folder. So if you don’t like /themes/mytheme/layouts/_defaults/index.html then create /layouts/_defaults/index.html and add your changes.