Structurizr retains previous versions of your workspaces (Free users can only access the previous 3 versions).
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 "Load version" button.
An orange version 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 "Revert to ..." button on the workspace summary when viewing an older version of the workspace. Alternatively, opening and saving a previous version of a workspace (in the workspace or diagram editor) will cause that version to be the latest version once again.