Structurizr retains previous versions of your workspaces (only the previous 3 versions are accessible for workspaces owned by a free account).
Viewing a previous version
To view a previous version, open the workspace summary page for your workspace, select a version from the dropdown list, and click the button. A label will be displayed when viewing content from a previous version of a workspace, to signify that you are not looking at the latest version.
Reverting to a previous version
To revert to a previous version, click the button on the workspace summary page when viewing an older version of the workspace. If you don't see this button, check that the workspace isn't locked. Alternatively, opening and saving a previous version of a workspace (e.g. in the diagram editor) will cause that version to be the latest version once again.