The sort order is the natural order of the list of modules in each position. So if you want to move a module from the bottom to the top, you will have to re-select the module from the list to the desire order.
There's no need to drag & drop as every drop-down module list in each position will list down all the modules on your store. Just select the one that you need to display in that particular position.
hmm... I see... however that's an stupid idea IMO. say I'd like to move a module at the end to the second row. I need to click on the drop-down menu of the second row and replace it with that module and repeat the same process for the rest which is time consuming and confusing. they could simply put a sort number in from of each row (or maybe a drag and drop feature) to make it easier and faster