software update

Back up your data, check your applications for compatibility, and depending on which MacBook Air model you have, click on El Capitan, High Sierra, Mojave, Catalina, or Big Sur.

The current version of Microsoft Word requires at least Mojave and a 2012 one. If yours is too old or you choose not to upgrade, try a product such as LibreOffice.

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An upgrade to a later macOS is possible; or for now, you may try an office-like suite

that does most of what 'MS 365 Office' does.. And that is free; an 'opensource' idea.

• LibreOffice for macOS:

Free Office Suite - Based on OpenOffice - Compatible with Microsoft

https://www.libreoffice.org/get-help/install-howto/macos/

• Feature Comparison: LibreOffice - Microsoft Office - from The Document Foundation Wiki

https://wiki.documentfoundation.org/Feature_Comparison:_LibreOffice_-_Microsoft_Office

Depending on the other considerations, the office-like Apple software

in a Mac could suffice. Unless shared with new MS Office/Word users;

where compatibility may have changed. (Pages, etc does OK.)

Otherwise, an Upgrade to newer macOS includes more than word/pages, etc.

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

This article includes various means to see what your Mac hardware

could be upgraded to; and has downloadable installers to get them.