Here’s how to customize the main gallery list order for an event.
From the host account, go to the main event view - where you'll see your event analytics. If you're in a gallery/album (viewing photos & videos), click the back arrow next to the gallery name to exit into the main event view.
Click the Menu button at the top right (purple circle with 3 white lines) to open the "Event Menu"
Select "Custom Gallery List Order" from the menu list
Click the arrows to reorder, then click the "Save changes" button