Un-install 32bit Ubuntu and Install 64bit Ubuntu?

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http://askubuntu.com – I had a Windows 7 operating system, I then wrote over that OS with Ubuntu 11.10. This heavily increased my disk space, but since i installed a 32bit version of Ubuntu onto my once 64bit OS my RAM was cut in half. How exactly do you uninstall the 32bit and install the 64bit when you have no Windows to go back to and no CD-ROM drive? (I installed Ubuntu with a flash drive) (HowTos)