The update procedure for your CMS depends upon the method you used to install Netlify CMS.
If you are using a package manager like Yarn or NPM, use their standard procedure to update. This is how both the Hugo and Gatsby starters are set up.
If you are using the CMS through a CDN like Unpkg, then that depends on the version tag you are using. You can find the version tag in the /admin/index.html file of your site.
^2.0.0, the CMS does all updates except major versions automatically.
2.0.1, 2.1.0, 2.1.2.3.0.0 or higher.~2.0.0, the CMS will do only patch updates automatically.
2.0.1, 2.0.2.2.1.0 or higher.