Basics: Editing objects
Any attribute of an object can be changed using the Style and Properties inspectors: its size, shape, color, position, and so on. When you select an object, the applicable inspectors become available; use the inspector that deals with the attribute you want to change. As you change the settings in the inspector, the object on the canvas changes.
For example, say you want to change the color inside a shape object (its "fill"):
Select the object. 
If you don't see the Fill inspector, select Fill from the Inspectors menu. Click the color well in the upper-right of the Fill inspector. 
Use the Colors panel to choose a new fill color for the object. 
Your object has a new fill color! Yeah!! 