Microsoft offers a tool called “MediaCreationtool.exe” allowing to upgrade your current windows version to windows 10.
With that in mind, whatever bit-version of Windows you are running on your about-to-be-upgraded machine that is the version of the download tool you need to download if you wish to do an in-place upgrade and not a clean wipe.
You can find the download tool here. Select the correct version, 32 or 64-bit, to match your current Windows installation.