Lumi is integrated with Microsoft Office 365. 

If you have an existing Microsoft Business or Student account, you can edit documents live and collaboratively online within Lumi.

The first time you try to open a document in Lumi, it will ask you to log in to your Microsoft 365 account. You'll only have to do this once. 

If you don’t have a Microsoft account you can still view, download and upload word documents to Lumi.

There are some differences in the controls in the online version to offline Excel. This article from Microsoft explains the differences between using a workbook in a browser online and in Excel offline.