To move a group of elements while keeping them in the same places relative to each other, select the whole group as described in Selecting model diagram elements, then click on any selected element, or inside any selected submodel, and drag. It must be a fully selected element (not a highlit link connecting a non-selected element) and only components inside the same submodel as the dragged component will be moved.
Movement will stop if any component in the group being moved collides with another component. In very complex models, moving large groups of components may cause Simile's graphics to respond slowly as it redraws highlit links and checks for collisions. If this becomes a problem, use the preferences dialogue to choose "Quick drag" as described in Preferences: Edit. This turns off collision detection and link updating during a drag, allowing the graphics to run at full speed. In this mode, link re-routing and collision detection are done only when the group is dropped at the end of the drag. If the move results in elements overlapping, or ending up in a different submodel from where they started, the selection will return to where it was at the start of the drag.