Recommended is to add the theme as a submodule.
cd themes
git submodule add https://....git
To update the latest version.
git submodule update --init --recursive
To update to a specific version (tag).
cd themes/hugo-theme-aws-community-day
git checkout v1.2.3
Hugo first looks in a local folder 'layouts' before it looks in your theme. That means if you want to change something quickly, you can simply copy a theme file to the layouts folder and make the change there. For example if you want to change something in the header of the page:
/themes/hugo-theme-aws-community-day/layouts/_default/baseof.html
To:
/layouts/_default/baseof.html