Using the mouse
Simply select the node to be moved using the mouse, drag it over the node which will become its new parent, and drop it to make it a new child node. As easy as that! When you are in a node's drop zone, the mouse cursor will look like this (slightly magnified):
In addition, if you hold the Shift key down while performing this action, the dragged node will become a sibling, not a child, of the target node and positioned below it. Holding down the Ctrl key will also make the dragged node a sibling of the target node but positioned above it.