Scuffs and spills happen. Clean your bag's exterior shell and interior nylon lining using a soft, damp cloth, warm water, and mild soap. It'll scrub right up. Be careful, water in excess can cause damage to the electronics.
When you're not traveling, slip the canvas dust bag over your suitcase before storing. If you have suitcases in multiple sizes, you can nest them inside each other to save space: Just unzip the left side of the larger suitcase and set the smaller one inside-with the wheels all facing the same direction-then zip the larger bag closed.