using xrandr

identify your devices

xrandr

Screen 0: minimum 320 x 200, current 1920 x 1980, maximum 8192 x 8192
LVDS-1 connected 1600x900+0+1080 (normal left inverted right x axis y axis) 309mm x 174mm
   1600x900      60.01*+  40.00 
   1152x864      59.96 
   1024x768      59.92 
   800x600       59.86 
   640x480       59.38 
   720x400       59.55 
   640x400       59.95 
   640x350       59.77 
VGA-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 510mm x 290mm
   1920x1080     60.00*+
   1680x1050     59.95 
   1400x1050     59.98 
   1600x900      59.98 
   1280x1024     75.02    60.02 
   1440x900      59.89 
   1280x800      59.81 
   1152x864      75.00 
   1280x720      59.97 
   1024x768      75.08    60.00 
   832x624       74.55 
   800x600       75.00    60.32    56.25 
   640x480       75.00    60.00 
   720x400       70.08 
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)


VGA-1 is the external monitor
LVDS-1 is the laptop's screen

Now, select the external screen above the laptop's.

xrandr --output LVDS-1 --below VGA-1

Set the screen as the primary one (so the notification area sits on it)

xrandr --output VGA-1 --primary