Do your work in Excel Online
Name your file
At the top, you'll notice that Excel Online automatically names your workbook Book1 (or Book2, or Book3...). To give it a more meaningful name:
Click the current name.
Type a name that works for you.
Anything you do in Excel Online — from naming the file, to working in the cells — is saved automatically.
Do your work
After you've named your file, you can do what you usually do: enter data, add formatting, type formulas, make charts, and so on. All features are available to you on the tabs at the top.
If you find the tabs are taking up too much space, double-click a tab and the ribbon collapses to give you more room.
Edit in the desktop app
If Excel Online is missing a feature you need, you can edit the file in the Excel 2016 desktop application. To switch to the desktop app:
Click Edit in Excel.
The Excel app will launch and open the file.
When your changes are saved in the application, they get saved to OneDrive. There's no need to do a "Save As" and re-upload the file.