How to mass check-in SharePoint library files?

Here is a way to check in multiple files at once:

First, if you don't have ownership of the files then go to library settings. Click on 'Manage files with no checked in version'. Select all and take ownership.

If not already available, create a view with filter as following: 'Check Out To' = [Me].

Go to content and structures from site settings. Go to the view you just created. Select all files and check-in using the action tab.