How change display resolution settings using xrandr ubuntu 14.04

just type xrandr, this will list all available monitors and possible modes/resolutions that you can use. you will see a star (*) on the current used mode. For instance:

>xrandr
Screen 0: minimum 320 x 200, current 2720 x 1024, maximum 8192 x 8192
VGA-0 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 340mm x 270mm
   1280x1024     60.02 +  75.02* 
   1024x768      75.08    70.07    60.00  
   800x600       75.00    60.32  
   640x480       75.00    60.00  
   720x400       70.08  
HDMI-0 connected primary 1440x900+1280+124 (normal left inverted right x axis y axis) 477mm x 268mm
   1920x1080     60.00 +
   1280x1024     60.02  
   1440x900      59.90* 
   1280x800      59.91  
   1152x864      75.00  
   1024x768      70.07    60.00  
   800x600       60.32    56.25  
   640x480       66.67    60.00  
   720x400       70.08 

To change it, just type:

xrandr --output HDMI-0 --mode 1920x900

this will change my second monitor (connected via HDMI port) to a new resolution.

there are more parameters you can set, like frequency, etc. chenk more details with xrandr --help