There is no "upgrade" from a 32-bit OS to a 64-bit OS. It requires a full installation.
If you are using the free upgrade from Microsoft, I believe you will only have the 32-bit option. If you are purchasing a new Windows license (DVD or download media), then there is no reason not to get the 64-bit version. If your system actually supports more than 4GB of RAM, you'll want the 64-bit version.
If your system does not support more than 4GB of RAM, then it's not going to matter which version of Windows 10 you get as (unless you purchase the retail version), you will be required to purchase a new license for a new system anyway.