Upgrading OS on Earlier Device

Hello!

I need help. I am trying to upgrade the OS on MacAir 2011, 11" (mid 2011) so that I can install Microsoft Office. Current OS is El Capitan, but now I am stuck on what to do next in order to get Microsoft Office. OR is there a Microsoft Office for earlier devices. I understand that I cannot upgrade to Mojave as MacAir '11 does not support the latest OS, so I am unsure about how to proceed...for an OS and Microsoft Office.

Yes...that is my dilemma. If there is any help available, please help me. Thank you.