You can't do an upgrade-in-place from any 32 bit version of Windows to a 64 bit version. You'd have to do a clean install.
The main downside to that is whether you have any pre-installed software that you'd like to keep. There would be no practical way to transfer that to the new Windows installation.
You may find this of interest: Clean Reinstall - Factory OEM Windows 7
It's aimed at getting a bloatware-free version of Windows, but it will serve in changing to 64 bit.