To edit a rectangle first select it.
A selected rectangle
Drag either of the rectangle's corner manipulators( 
) to change the corners or ends.
Drag any rotate manipulator 
to rotate the rectangle.
Drag any scale manipulator
to re-size the rectangle.
Drag the rectangle to move it.
Rectangle Properties Dialog
You can also edit the parameters of the rectangle using its properties panel as shown below. To view the properties panel first select it.Then right-click on it, then select Properties Panel from the context menu that opens.
Rectangle Properties Dialog
X
The X coordinate of the center of the rectangle.
Y
The Y coordinate of the center of the rectangle
Width
The width of the rectangle.
Height
The height of the rectangle
Roundness
Drag the slider to change the roundness of the rectangles corners.
Layer
Use this drop-down to set the layer for the rectangle.