Do you have Windows 7 or 8?
If so, and it's legit then you can get a free upgrade to Windows 10. When you do that, make sure to do a CLEAN INSTALL using the 64-bit version. Plan carefully to save data, write down program names etc.
You SHOULD be able to do a clean install of the W10 64-bit Preview version then simply validate it after July 29th to transfer over your license code for W7/W8.
*I'm reasonably certain you can upgrade from 32-bit to 64-bit in the sense that you won't be forced to download ONLY the 32-bit version of Windows 10 but again you have to do a CLEAN install.