I use Ubuntu 12.04 LTS. I tried to upgrade it to 14.04 by booting from a usb stick. In the menu there were just two options - one was to install 14.04 by erasing 12.04 and another was to install 14.04 by erasing everything. But problem is both of them said that my documents, downloads and musics will be erased totally. Is there a way in which I can upgrade my OS to 14.04 without losing any data?
Yes, but not from a usb stick if I understand correctly. You will have to get online to where you can upgrade directly from the ubuntu servers.