1. Click on the Grid view icon in the top right of the toolbar
2. Click the Reorder button in the top right
3. Drag and drop the books into the order you want them.
By default, the books are ordered by creation date, with the most recent in the top left.
4. When finished, click the Done button.
Here's what it looks like:
Note - if you have published the library, this new order will automatically be reflected in the public version of the library.