How do I undo the update?

I just updated the software my iPod Touch 4G 8gb and I now only have 3gb free to use which does not at all fit my music in here. I already have a little bit of space since it is only 8 gb but I do not want to buy a new one. I want my memory back. I paid for 8 gb, not for 3, and I want as much of that back as possible.