Elsewhere, users can now also access password-protected workbooks on Excel for the Web, meaning there's no need to switch back to a desktop in order to open & interact with secured files.

Click on the Excel "File" tab. Click the "Open in Excel" option to launch your desktop version of Excel. Click "OK" at the Internet Explorer prompt that begins, "Some files can harm your computer." ...

If Excel Online is not working and you see errors Couldn't Open the Workbook, or Request Was Taking Too Long, then see this post for the fix.

Then, I’ll present a quick way to force Office 365 to always open a file in the desktop environment, regardless of where you open or save it. I’m using Office 365 on a Windows 10 64-bit system.