boot - Replace one Ubuntu with another from USB pen drive

I'm new to Linux. I'm using Ubuntu 11.04. Now i want to install Ubuntu 12.04 .

I got an ISO image of Ubuntu 12.04 Desktop.
I put this image in to a pen drive which is formated,set the boot option boot from usb but nothing happened .

I searched this over the net and on Ubuntu website but nobody has given the complete steps . someone say you can install from the Ubuntu also ,someone says u can do a fresh installation from usb pen drive you need to make you pen drive bootable etc. etc. .

My problem is that i don't know the exact steps how ton install Ubuntu from usb pen drive?

All I want to do is to completely remove my Ubuntu 11.04 and install Ubuntu 12.04 from usb pen-drive.

Can any body tell me how to make a pen drive bootable ?

How to install Ubuntu 12.04 from pen-drive?

Please give me a step by step procedure with screenshots.

Thanx in advance

I think there is no easier way than this illustrated here.

In short:

• After Downloading Ubuntu.




• Open the dash and search for Startup Disk Creator (for the dash click the icon on top right).
  



• Start the Startup Disk Creator and Click on Other
  



• Then select open and search the location that you have downloaded Ubuntu (normally in ~/Downloads)




• Select the USB stick in the bottom box and click 'Make Startup Disk'.
  

Taken from this excellent guide.