Files can also be checked out by clicking on the ellipses  to the right of the file name, selecting More then clicking Check In.

Site or library owners can check in on behalf of other users. This is helpful if the user who checked out the file is away. This commits any changes that were made to the file by the user who had checked it out.

If you do not want to commit the changes made to a checked out document, you can use the Discard Check Out option.

Retain Check Out will allow users to see the latest changes to the file but keep it checked out under your name.

Libraries with Versioning will allow you to choose to check in a Minor Version, Major Version or Overwrite the current minor version.