The 2D drawing views that you create on a layout are associated with the source 3D model. When you make changes to the source model, various alerts on the layout indicate that the drawing views are out of date. Red corner glyphs display around all the affected views and a bubble notification extends from the Drawing Views icon on the status bar.
You can easily update all the affected views by selecting a link on the bubble notification. If you want to limit the updates to selected views, you can use the View Update tool, available in the ribbon and right-click menu.