How to move existing media files fro the root of the uploads folder to subfolders or date structured folders?
Normally, WordPress stores all your images and other media files in the /wp-content/uploads/ folder. To keep it structured, all files are stored in folders organized by year and month.
The problem you will run into with reorganizing them is that links to the files in posts and pages will not update if you are simply moving stuff around the server. First, you need to install and activate the Media Library Folders plugin. For more details, see the plugin description and installation guide.