This is what you do to install Ubuntu from USB.
First you have to download the iso from Ubuntu.
Then you need to download Universal USB Installer.
Now you need a USB-stick larger then 1 Gb.
Run Universal USB Installer, click I Agree and set your settings.
Step 1: From the drop down menu select your Ubuntu version
Step 2: The browse for the iso you downloaded
step 3 : pick your USB-stick. you will have only one to pick form if you have only one in the computer.
Now just click create. This will take some time.
And you are ready to install Ubuntu form USB, just remember to boot from USB Device and the follow the instructions