As it is an ASCII file, you may edit the file directly in your favourite editor. However a much easier approach is to type use the oracdisp GUI (simply type oracdisp). This has various options available at the top and shows the disp.dat entries corresponding to those choices at the bottom. Don't forget to write press the ``Configure'' button to write the file to disk.
You may use oracdisp (or the editor) to change the display parameters while the pipeline is running.
ORAC-DR: Overview and General Introduction