Fork the source github repository (button in upper right)
If it’s a small change, then you can navigate to the relevant page in your github repository. (Any fiel you want to change will be in a subdirecotory of the content folder.) Then you can click the Pencil/edit button to edit the markdown file, and click “Commit changes” to commit it to your repo.
If it’s a larger change, then clone your github repository to your computer, make the additions/changes (again should be in a subfolder of content), commit the change.
(Ideally, you’d test it yourself on your own computer first. If you’ve installed hugo, then you can run hugo server in the root directory of your local repository and browse to the link provided to view the new site. That said, I’d rather have more contributions with useful content and formatting imperfections than fewer contributions that are beautifully formatted.) Then push your commites to your repo on github.
Create a pull request for me to merge your changes into my github repository.
Wait for me to merge it (or ask you to make changes) and netlify to regenerate the site.
Make sure your changes didn’t completely break the site.