If you want to upgrade to 64-bit versions of either Office 365, or the single-purchase, perpetual license Office 2019 for Mac, you will have to pay. That lifetime student license you purchased six years ago does not imply free lifetime updates when the application suite is retired and no longer supported.
Your only practical course now is to download the free, 64-bit LibreOffice Office suite, which is an MS Office Suite clone. Your documents will open just fine. There is a Documentation link from the LibreOffice Get Help menu, and since it is downloaded outside of the Mac App Store, you will need to follow directions in Safely open apps on your Mac, to get LibreOffice square with Catalina security before you can use it.