From mid-Januari 2019, Office ProPlus and Office 2019 will now be installed with 64-bit as the default setting. The 64-bit version of Office will automatically be installed unless you explicitly select the 32-bit version before beginning the installation process.
Instructions for user initiated installation from scratch (Office 365): https://support.office.com/en-us/article/download-and-install-or-reinstall-office-365-or-office-2019-on-a-pc-or-mac-4414eaaf-0478-48be-9c42-23adc4716658#InstallSteps=Install_on_a_PC
Important stage from the guide above: You can then select the version you want. Select Other install options, your language, and then under Version select Advanced, and choose 64-bit or 32-bit, and then select Install.
Instruction to change from 64-bit to 32-bit version:
Note: If you install the 64-bit version, but wanted the 32-bit version instead, you must first uninstall the 64-bit version before installing the 32-bit version.
Instruction how to uninstall: 64-bit Office: https://support.office.com/en-us/article/uninstall-office-from-a-pc-9dd49b83-264a-477a-8fcc-2fdf5dbf61d8