From the ITEMS tab of the auction, you can click into the first column, the Display Order, and change the order of the item. All existing display order numbers will resort in ascending/sequential order.
You can also add items in between existing items. Click on Add to add a new item, or edit an existing item, and change the display order # and the item # as seen below. In this example we are creating an item#1A that will go in between existing item#'s 1 and 2.
The result is seen below:
Alternatively, you can do this in bulk by downloading the MS Excel inventory file for this auction and editing the Display Order and Item Numbers directly and then save/upload.