This is what the Help files tell you.

1.

First ensure that the second monitor is attached to your video
card and is turned on.

2.

Click the icon at the very right of thetop paneland selectSystem
Settings.

3.

In theHardwaresection, selectMonitors.

4.

Click the rectangle that says your monitor's name, such as XYZ
Brand 23. To enable it, change the setting at the top of the
dialog fromOfftoOn.

5.

Select the appropriate resolution that your monitor supports,
refresh rate, and rotation (if necessary). If you are unsure which
settings to use, then refer to your monitor's manual.

6.

If the monitor's virtual location does not match its actual
location, drag the monitor's rectangle around to the appropriate
location.

7.

To test the new configuration, click theApplybutton. If it looks
correct, clickClose.