Update

You might check your sources for system requirements for MS Office 365.

I believe the current version requires macOS Mojave or newer.

https://support.microsoft.com/en-us/office/upgrade-macos-to-continue-receiving-microsoft-365-and-office-for-mac-updates-16b8414f-08ec-4b24-8c91-10a918f649f8

How to Upgrade  macOS:

The macOS version you can upgrade to depends on the year your Mac was released.

Click the  (Apple) menu at the top left of your screen and choose About This Mac.

❗️ – IMPORTANT – Backup your data, now, before you attempt an upgrade.

Learn How to Back up your Mac with Time Machine:

https:/https://www.stclairsoft.com/Go64/

❗️ – IMPORTANT – NONE of your 32-bit apps will work in Catalina or Big Sur.

https://support.apple.com/en-us/HT208436

Before you upgrade, use this free utility to check the compatibility of your apps:

Go64: https://www.stclairsoft.com/Go64/

🖥 – Check the compatibility of your Mac with newer versions of macOS.

https://support.apple.com/en-us/HT201475

✅ - Using Safari only, download the macOS upgrade installer app:

Click > High Sierra, Mojave, Catalina or Big Sur.

Once downloaded, double-click the installer app in the Applications folder to begin the upgrade.