In order to reorder images in the Library, view them in Grid view and then drag (in the thumbnail) and drop in order to reposition. This creates what’s called a Custom Sort order. However, Custom Sort orders depend on folder hierarchy. If a folder contains subfolders a custom sort order is not possible unless the option to “Include Items from Subfolders” is disabled (in which case you will only be able to see the photos in the targeted folder).
And don’t worry, you can still use the Sort option in the tool bar to sort by other means (like Capture Time and File Type) and when you return to the User Order, Lightroom will remember your Custom Sort Order.
If you want to reorder images that have subfolders (without moving them from their original folders), then select all of the images and create a Collection (not a Smart Collection – Custom Sort orders are not available for Smart Collections) and then reorder them in the collection.