As was stated earlier, it depends on what you mean by "downloaded". If Ubuntu is installed, once you are on the Unity desktop, you should see an icon with a spiral pattern on the upper left part of the screen. Click on it, or alternatively, press the Super key (the one with a Windows icon). Once there this is done, in the search bar that will appear, type "LibreOffice". It is a suite very similar to OpenOffice. Click on the LibreOffice icon to launch the program.
If Ubuntu is not installed, you first have to create a bootable USB stick. You can do so under Windows using a program called Pen Drive Linux. For instructions, search for "Ubuntu bootable USB".
Once this is done, shut down your computer, enter the USB key, and start your computer. Look for a message mentionning a key to press to access the BIOS, which contains boot options. Once there, choose to boot on the USB key.
Once you have booted on the USB stick, you can install Ubuntu on your computer. Once installed, you will be able to use the method I talked about above. You can also work directly on the USB key without installing Ubuntu on your computer, which is called the Live version, but if you are to use it often I suggest installing it.