Note: The 64-bit version is installed by default unless Office detects you already have a 32-bit version of Office (or a stand-alone Office app such as Project or Visio) installed. In this case, the 32-bit version of Office will be installed instead.
To change from a 32-bit version to a 64-bit version or vice versa, you need to uninstall Office first (including any stand-alone Office apps you have such as Project of Visio). Once the uninstall is complete, sign in again and select Other install options, choose the language and version you want (64 or 32-bit), and then select Install.