You can rename a folder, link, or file in your Office 365 document library.
Note: When you rename an item that you've previously sent someone a link to in your document library, you'll need to send them a new link.
Rename a document, folder, or link in a document library
Open the document library and hover over the file you want to rename.
Click the ellipses (...) to the right of the item name, and then click Rename.
In the Rename dialog, type the new name into the field, and then click Save.