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The more you use Sites in Office 365, the more likely you’ll need to copy or move documents around, either between your desktop computer and a site, or from one site to another. Uploading files gets easier after your sites are upgraded to the new SharePoint Online experience, thanks to new drag and drop functionality. (To learn more about the service upgrade, see Upgrading to the new SharePoint Online.)
A new video on Office.com shows how to move files between sites whether or not your site has been upgraded to the latest user interface.
Note: Using File Explorer to move SharePoint files does not preserve metadata, such as the original creation date, unfortunately. If preserving the original file metadata is important to you, consider searching the Office 365 Marketplace for a file migration solution available from one of our partners.
Some users in the Office 365 Community have had issues with Open with Explorer recently, and we are working on solutions to those. In the meantime, for additional help with Open with Explorer, see Troubleshoot the 'Open with Explorer' command in Microsoft SharePoint Online or search here in the Office 365 Community forums.
--Matt Evans, SharePoint Writer