No internet,NO cd/dvd rom, only USB. I have no idea how to upgrade the system. Any one can tell?
I THINK the only way to do it would be to get someone with Internet to follow these instructions for 13.04 and then you will have to put it into your PC, reboot to the bootloader (you might need to press ESC or F5 or something while booting to get to the Startup Menu in your PC) and then select USB. You can then select the option 'Upgrade to 13.04' to keep your files.
You might want to back-up your data maybe by simply copying the files in your Home Folder to an external hard-drive and then copying them back if you need to.