You will probably need to reinstall your original OS, and then upgrade it again to the 64bit version, but you can try just doing a clean install of the 64bit version first as that may work as well.
Was your original OS a 32 or 64 bit version? If your qualifying version of windows was 32 bit, you can only upgrade to 32 bit so far as I am aware of.
How to do a clean install
Dowload the ISO here:
https://www.microsoft.com/en-us/software-download/windows10
Use this tool to create USB or DVD installation media using the downloaded ISO:
https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool
Windows 10 clean install tutorial:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html