This information applies to the latest version
macOS:
DxO Standard is the default workspace in DxO PhotoLab, selecting this from the "Workspace" main menu will revert to DxO PhotoLab default.
Windows:
Open the "Workspace" main menu and choose either of the default workspaces: "DxO Standard" or "DxO Advanced". This menu item is accessible only from the "Customize" tab.