How can I use the keyboard on my MacBook to switch between open Word documents?

Apple has a long and daunting list of keyboard commands in the support document Mac keyboard shortcuts - Apple Support

To switch between open windows of one application:

Command–Grave accent (`): Switch between the windows of the app you're using. (The character on the second key varies by keyboard. It's generally the key above the Tab key and to the left of the number 1.)

Have a look what keyboard combination is set on your Mac for this action.

Go to your Settings > Keyboard > Keyboard Shortcuts > Keyboard

Towards the bottom of the list you will see Move focus to next window. On my Mac you can see that the combination is the command and `

What is the setting on your Mac?

What macOS version?

I just tested on my 2024 M4 Pro Macbook Pro running latest macOS and fully updated version of Office 365 and the "command `" works without issue witehr oprn Word docs.

On my U.S. keyboard layout, the ` key is this one:

This keyboard shortcut did not work for me in the past (even though it should have.) Go to System Settings > Desktop & Dock and turn on and off Stage Manager. Reboot the Mac for good measure.