Articles live within the tree structure of a Collection. In this way, Articles have parent, child and/or sibling relationships. This most obvious way in which this is exposed is in the creation of new Articles, when you will select whether the new Article should be "next to" (a sibling of) and "below" (a child of) the current Article.
You can move Articles within this tree structure by following these steps:
Open the Article you wish to move
From the Article Options menu (right-most green button on the top of the page), select "Edit"
While editing the Article, click the orange Move button (two crossed arrows icon) on the top right corner of the page.
Choose the new destination. Be careful to select a destination you have access to, otherwise you may lose the ability to view/edit the Article after it is moved.
Optionally update the Article slug
Optionally have redirect pages created in place of the Article at it's original location.
Click "Move Article"
Note: Any links from other Articles that point to the original location of the Article will not be automatically updated. As such, it is a good practice to create redirect pages if the Article has lived in its original location for a while and/or has many incoming links form other Articles.