Just enter xrandr to get hold of the display names. Then enter xrandr –output <display name> –primary i order to make your external screen the primary one. Done! See example below:
magnus@gollum:~$ xrandr
Screen 0: minimum 320 x 200, current 3600 x 1200, maximum 8192 x 8192
VGA-0 disconnected (normal left inverted right x axis y axis)
LVDS connected 1680×1050+1920+0 (normal left inverted right x axis y axis) 331mm x 207mm
1680×1050 60.6*+
1400×1050 60.0
1280×1024 59.9
1440×900 59.9
1280×960 59.9
1280×854 59.9
1280×800 59.8
1280×720 59.9
1152×768 59.8
1024×768 59.9
800×600 59.9
848×480 59.7
720×480 59.7
640×480 59.4
S-video disconnected (normal left inverted right x axis y axis)
DVI-0 connected 1920×1200+0+0 (normal left inverted right x axis y axis) 518mm x 324mm
1920×1200 60.0*+
1600×1200 60.0
1280×1024 75.0 60.0
1152×864 75.0
1024×768 75.1 60.0
800×600 75.0 60.3
640×480 75.0 60.0
720×400 70.1
magnus@gollum:~$ xrandr –output DVI-0 –primary
magnus@gollum:~$
