You'll have to arrange them manually, I'm afraid; there is no automatic sorting. To do:
Open your Bookmarks (by pressing on the open book icon). Press the button marked "Edit" at the top right of the pop-up. Then, your view of your bookmarks will change and you will see three gray lines to the right of every bookmark. Press and hold on one of them and, still holding, move the line up or down to arrange it as you wish; rinse and repeat with any other bookmarks you have. When you are finished, press the blue Done button (upper right, again) and you are finished with your bookmarks all sorted.
Hope this helps. Let us know how you get on.
Marilyn