You can change the order of the items in a gallery in two ways, either through the manage menu or through the settings panel when you have a gallery item selected.
Using the manage menu:
1. Select the gallery that you want to work on.
2. Click Manage Gallery in the settings panel.
3. Select an item and then use the arrows next to it to change it's place.
Using the settings panel:
1. Select the gallery that you want to work on.
2. Select the individual item that you want to work on.
3. Use the arrows in the settings panel to change the item's position.