If you installed by Git cloning hugo-nsd
Before updating for the first time, the remote origin repository should be renamed to upstream:
$ cd themes/hugo-nsd
$ git remote rename origin upstream
To list available updates:
$ cd themes/hugo-sd
$ git fetch upstream
$ git log --pretty=oneline --abbrev-commit --decorate HEAD..upstream/master
Then, update by running:
$ git pull upstream
Troubleshooting
Check out the release notes for the version that you are updating to, paying attention to the breaking changes.