The background update brings improved reading experience, extra options for file management, OneDrive integration, proofreading tools, and more. And because these web-based productivity apps are only available in the browser, there is no need to update anything - you'll get the new features on a rolling basis.
The update to Office Online brings the following improvements according to the official changelog.
- Enhanced Reading Experience: Reading View has got a new toolbar with Edit, Print, Share, and Comments commands just one click away. More advanced tools, such as Translate or Download a file, are also easier to find.
- More options to save and manage your files: Microsoft has expanded Save As options to provide you additional file management options that we think you will appreciate. The Save As options now include saving a copy to your OneDrive, Rename, Download a Copy (if it’s stored on OneDrive), and Download as PDF which converts your file format to a PDF and then gives you the same choices as “Download a Copy.”
- Add files to OneDrive from the toolbar: You can now make easily make copies of documents you couldn’t open for editing (view-only) that will be saved to your OneDrive with a new command that can be found on the toolbar.
- Get started editing faster: The new start experience is optimized to get you into the right editing experience as fast as possible. It’s now easier to navigate through your most recent documents from your device or from your OneDrive, and it’s now faster to open a blank document or a template.
- Integrated Help: Now you have a Tell Me search box on the ribbon so you can find what you’re looking for more easily.
- Improved Proofreading Tools: Now there’s a highlighted word count feature, and the proofreading service is improved.
- Office On the Go: After installing the new Word, Excel, and PowerPoint apps on your device, you can open a document from Office Online in the corresponding Android app to have more functionality, the same way you can already do on the desktop.
source - Microsoft