Resolution

This program allows you to change the screen resolution at any time.
The change is not permanent. This is useful when connecting a laptop to
an external monitor or a projector, for example, which may require a different
screen resolution.

Resolution is simply a graphical front-end to the xrandr (X Rotate And Resize)
command-line utility, which you will also need. Modern X servers (such as
XFree86 and X.org) come with this program. If you don't have it, try upgrading
your X server.
Note that there is another, older, mechanism for changing resolution, often
activated by pressing <Ctrl>+<Alt>+<Keypad Plus>. This still changes the
resolution, but your desktop remains the same logical size (i.e. you 'zoom in'
to the screen and must pan around to see your panel, etc).