The full free theme doesn’t live in the WordPress.org directory, so it doesn’t update through the directory the way most themes do. Canvas handles updates with a bundled GitHub update checker instead.
The checker watches the GitHub releases and surfaces an update notice in wp-admin when a new version is published, in the same place you’d see any other theme update. You review it and update from there, and the new release is pulled from GitHub.
This gives self-hosted installs the same update experience as a directory theme, without the directory. If a notice isn’t appearing when you expect one, see the update troubleshooting article.