If you have generated contour drawing with one interval, say 0.5M and if you want to generate contours with different interval say 0.25M. Then it can be done in 2 different ways.When contour drawings are generated it will create the drawing entities in different layers. That is major contour, minor contour, grid line, elevations etc. All of these will be in different layers. If you want to generate drawing with a different setting, you have to delete each of these layer by layer.Using 'DSL' command you can delete all the entities of the selected entity layer. With this command, you have to run multiple times and you have to clear layer by layer.
Note: If you give UB command, all entities generated with contour map will be deleted at once. If you have edited the drawing after contour generation and if you give UB then not only entities related to contour, but also additional work done on the drawing will also be deleted, if you have made changes in drawing then better to us DSL command. Whereas, if you wish to immediately get back the drawing to a stage before generating contour, use UB command.