In document libraries, when you click on a document’s name, the Office program might unexpectedly open it as a read-only file. Here’s how to open it for editing.
Instead of clicking the document title, click Open Menu (the three dots), and then click Edit:
If you’d rather have documents open for editing in Office Online when you click the title, you can set up the library to do that (as long as you have permission to change the library settings):
In the library click the Library tab. Go to Library Settings > Advanced, and switch to Open in the browser in the Opening Documents in the Browser section.