From the comments, if you've got the 32bit version installed and want to install the 64bit version (basically, it's not an update from one to the other): – Uni

From the comments, if you've got the 32bit version installed and want to install the 64bit version (basically, it's not an update from one to the other): – Uninstall Win32 – Don’t remove your profile – Install Win64 (it’s a full installer vs. a update)