The problem is you have to be root user to change the display settings. You can do this two ways:

1. Logout of KDE and login as root user, then try to change display settings using KDE control center.

2. To do it without logging out, open a terminal and enter "su", then you will be prompted for a password. Enter the root password, then you will be root in the terminal. Enter the command "kcontrol" to open the control center as root and make the display change.

Good luck.