SharePoint Online - Document Library Settings to rename the name.

When you are in a SharePoint Document Library, and you click on the Gear Icon on the Top Right Hand Side Corner. It will open up a side panel, from where you can rename the Document Library as well, and add/ change the description for the Library itself.

- To access the traditional library settings page, you can click on the “More library settings” link.

Thanks, @SharePoint Wizard for the tutorial