Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
In theory, you simply have to uninstall and re-install.
Starting earlier this year, in the spring, MS finally made 64bit the default.
Uninstalling Office does not affect your personal files. Doing a simple uninstall from the Start menu, Programs and Features or Settings will leave your current configuration information in place, ie Word NORMAL.DOTM.
If you explicitly want to ensure you install 64bit you can download the 64bit "Offline installer".
Use the Office offline installer (MS)
The Offline Installer should allow you to install Office without an internet connection (after the file is downloaded). This page also gives you the option to explicitly select 32 or 64 bit installation.