Right-click a room or object. In the menu that comes up choose Bring to Front or Send to Back.
Use the "Bring to Front" tool to bring the selected object to the front layer.
Use the "Send to Back" tool to send the selected object to the back layer.
Graphics and CAD images are exceptions in that they work across all rooms regardless of layers. In other words, the software will not send a room behind the locked graphic layer. This will help avoid situations where rooms "disappear" when they could be behind the graphic.