Plug in the USB flash drive and determine the device it's mounted on with the command: 

sudo fdisk -l
This time around it was /dev/sdb for me, so I'll use that as my example but it may /dev/sdc too. Almost all of the time, it can not be /dev/sda 

umount /dev/sdb
Then Open a terminal with Ctrl+Alt+T shortcut and use this command. I'm assuming your USB drive is /dev/sdb

Using dd:
You can also use the dd command. Open a terminal use a command like below: 
 
sudo dd if=/path-to-the-iso-file/filename.iso of=/dev/sdb
here path-to-the-iso-file is the path where the *.iso located and filename.iso is iso file name. The command will run for some time. When you see the prompt $ in your terminal, you're done.

Using cat:
You can also use the cat command. Open a terminal use a command like below: 

sudo cat /path-to-ubuntu-iso/iso-filename.iso > /dev/sdb; sync
Replace the /dev/sdb with your system specific USB drive number. 

 Please note that, You need to specify the output device as /dev/sdb or /dev/sdc not /dev/sdb1 or /dev/sdc1 etc.