I am trying to upgrade to MacOS Big Sur but can't figure out how.

As "dialabrain" pointed out the latest macOS version supported by your iMac is Catalina (10.15.7)

To update High Sierra to Catalina go to the Apple Menu and select System Preferences.

In System Preferences click on the Software Update icon and follow the prompts.

It's always a good idea to have a current backup before updating your Operating System.

Open in Safari: macOS Catalina 10.15

Catalina is the latest version you can run. See: How to get old versions of macOS - Apple Support

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/