If your Brother machine's Copy Quality setting is Best and you cannot change it, you must change the machine's Paper Type setting to Plain.  After making this change, you will be able to change the Copy Quality setting.

Changing the Paper Type setting

To change the machine's Paper Type setting to Plain, follow these steps:
 

1. Press (Settings).
    
2. Press the Up or Down arrow key to display General Setup.
    
3. Press General Setup.
    
4. Press the Up or Down arrow key to display Tray Setting.
    
5. Press Tray Setting.
    
6. Press Tray #1

   You cannot use plain paper in Tray #2

7. Press Paper Type.
    
8. Press the Up or Down arrow key to display the paper type options..
    
9. Press Plain Paper.
10. Press (Home).