On the download page,
https://www.calibre-ebook.com/download_windows the top link downloads the 32 bit version. If you copy the link and paste it into your browser's address box you can change the win32 to win64 and get the 64 bit version. The first link below that, location #1, gives you a list for the different platforms and from there you can select the 32 or 64 bit version. For location #2 you'll need to do more url fiddling to get to the proper github page where the different platforms and bit versions are available.
Anyhow, from the main windows download link one might get the impression that the 32 bit version is preferred?