NOTE: Full Control or Designer permission level is required to be able to perform the steps below.
1. Navigate to the document library. At the bottom left of the page, click Return to classic SharePoint.
2. Click the Settings (gear icon) > Edit page.
3. Under Page tab, select Make Homepage.
4. In the prompt window, click OK.
5. You should see This page is now the site homepage.
6. Still under Page tab, click Stop Editing.
7. Below the Quick Launch (or side navigation), click Exit Classic Experience.