You cannot currently move files from one folder to another in File Manager. If you need to move a file to a new place, you will have to download it and delete it, then re-upload it to the new location. Or, if you have the file on your local computer, you can delete it from the current folder and upload it to the new one.
Please be aware that changing the location of a file will result in broken links on your website wherever that file is currently linked to on your site. Hyperlinks to files are based upon folder path/location, so moving files to another folder will break those links.
To find out whether any links will be impacted by moving or deleting a file, select the green "Where used?" icon for the file on the right side of the file list.
This will display a list of all the components of your site and whether that file is being used in any of those locations. Click on the blue number to reveal links to each of the places where the file is in use. Take care to visit each of these locations and remove the links to the file before you delete the file in File Manager, or update the links when the file has been uploaded to its new folder.