Sur support for MacBook Air 11" Early 2015 OS High Sierra 32-Bit MS Office Suite

32-bit apps will not run on Catalina or Big Sur.

Microsoft does not provide free upgrades between major versions of their Office products (unless you subscribe to Microsoft 365) and older versions of Office are not compatible with macOS Catalina or Big Sur.

If you have a version of Microsoft Office that is not compatible, you can either subscribe to Microsoft 365 or you can make a one-time purchase. See: https://products.office.com/en-us/buy/compare-microsoft-office-products-for-mac?ms.officeurl=mac-compare-redir

Or use the free and very good Libre Office https://www.libreoffice.org/

or the free Microsoft Office on the web: https://products.office.com/en-us/free-office-online-for-the-web

or Google Docs: https://www.google.com/docs/about/

or Apple’s Pages and Numbers

FWIW, no version of macOS later than Mojave 10.14 supports 32bit apps.