We are really pleased that, this launch of SharePoint 2016 brings back the earlier removed "check in" and "discard check out" feature in the document library. Since these features were gone the only way to do it was to revert to the classic SharePoint view. This made our customer confused and meant had we needed to recommend setting the classic view as the default.

If you want to make changes to a file in a SharePoint Online document library, but keep others from making changes at the same time, check the file out of the document library. When you check out a file from a SharePoint Online document library, only you can edit it. You can edit the file either online in Office 365 or offline and save your changes as often as you like before checking the file back in. Once you're done making changes to the file, check it in from the library to upload your changes. You can discard your changes and undo the check-out if you change your mind.

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