There are 2 ways to edit a coordinate, dragging it or using the Properties panel.
Dragging
You can hold down the left mouse button on a coordinate to select it and then drag the mouse to move it. You will see the X and Y values of the coordinate change as you move it. Press the 's' key to turn snap on or off.
Properties Panel
If you select a coordinate and the Properties panel is visible, the coordinate's properties dialog will be shown.
X
The X or horizontal position of the coordinate.
X
The Y or vertical position of the coordinate
Pressing the 
button displays this help page.