cb-display-tool

If you are using lubuntu, try the following commands

#wget http://dl.cubieboard.org/software/tools/armhf/cb-display-tool
#chmod 777 cb-display-tool
#cb-display-tool -o 8 -m 11  #switch to VGA type
#cb-display-tool -o 4 -m 4  #switch to HDMI type

If you switch to other resolution, you need to restart your X after the modification