(1) Click on up and down of the order controls or change the value there.
(2) Use the green "OK" button to write the changes to the database
See following image
The sorting depends on parents and child galleries. Child galleries will be ordered direct behind their parent. This applies also to sub - childs when the parent is a child itself. Then a cascade of childs and sub childs will follow the first parent.
(3) While changing the "first gallery" from 5 down to 2 the child gallery "a subgallery" will always follow having a ordering with 1 higher. Result see below
Sadly this has a side effect. If you try to increase the parent gallery ordering it will not go up through clicking. You may set te value by key which works when leaving the control.