Do you want to install Office 365 client on MAC OS? Here is an article about system requirement for Office 365 client, you can have a look about it.
You can login your Office 365 account center: https://www.office.com/. if you could see and download, you may could install Office 365 client:
If you cannot install Office 365, you could contact Apple Official Support to change the version of your MAC.
By the way, this tag is for Exchange online server side, if you have more question about Office 365 client, you could post new question with office-itpro tag。
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