In the left tree, click on the folder , hold down the mouse buttonand move up / down the element in the tree.
One page can be sorted inside its parent. If you want to change its parent, edit the page and set its parent page in the right options panel.
Sort an article through the same process than a page, by drag'n'drop after mouse hold on the icon .
Articles can also be sort in the page edition panel : Edit the parent page and sort articles in the articles list by drag'n'drop.