Make a standard bootable Ubuntu USB from within pre-installed Ubuntu (simple)
Frist, check the installed version of Ubuntu by opeining a terminal (press Ctrl+Alt+T) and type
Note down the version number and OS Type (32bit or 64bit).
Second, download the ISO from http://www.ubuntu.com/download/desktop, by selecting the appropriate version.
Third, insert an USB drive (1GB or greater, 2GB or greater if you want to make it persistent).
The Startup Disk Creater takes two inputs, the location of the ISO file and the location of the USB drive.
Fourth, point the Startup Disk Creater to the ISO and the USB drive.
See the help on Create a usb stick on ubuntu for a simple set of instructions.
Also see a more detailed instructions on USB stick creation
Make a custom bootable backup Ubuntu USB from pre-installed Ubuntu (Complicated)
First, Add Remastersys source information to Software Center. Open Software Center and click on “Edit” and select “Software Sources.”
Select the “Other Software” tab at the top and you need to click the “Add” button, and copy and paste this:
deb http://www.remastersys.com/ubuntu precise main
And then click “+Add Source”, then click "Close" and completely close out of Software Center.
Add the PGP key for the Remastersys repository, in Terminal (press Ctrl+Alt+T) copy the two lines below one line at a time and paste (press Ctrl+Shift+V) in Terminal:
wget -O - http://www.remastersys.com/ubuntu/remastersys.gpg.key | sudo apt-key add -
sudo apt-get update
Second, install Remastersys. Open Software Center and search for and install remastersys.
Third, run Remastersys. Click on "Backup" and wait a long time till it finishes creating the ISO file.
Fourth, create startup USB following instructions above.
More detailed information and some qualifications on how to do this is here.
Hope this helps.