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 Menu.
2. Press the Up or Down arrow key to select General Setup.
3. Press OK.
4. Press the Up or Down arrow key to select Paper Type.
5. Press OK.
6. Press the Up or Down arrow key to select Plain Paper.
7. Press OK.
8. Press Stop/Exit.