You’d almost certainly want to start from the repo rather than modifying the output files from the site you’ve already built.
Obviously you can do what you prefer, but if I were you, I’d be working to troubleshoot your local hugo build since then you can change the source code following the recommendation in the hugo docs (https://gohugo.io/documentation/). Since you have a data source of forestry, any changes you make outside of git will not persist with your next content update via forestry, which seems pretty suboptimal to me!
So, while Netlify’s tech support team can’t really advise on how to update your local build environment in depth, our other customers can and we’ll certainly take a shot at advising at a high level once you share what specifically happens when you try what specific steps locally - I’d recommend pretending you have a new laptop, and saying something like:
“I installed Go version X, and node and npm versions Y and Z. I then tried to clone my repo using this command (quote command), and then tried to build my site from this repo (link to repo) using this command (quote command) and got this error (quote error message)”
That will give us more to work with when trying to help you troubleshoot. Better to overshare in all circumstances as long as it doesn’t include any secret credentials like password, API Token, or credit card