As you made your prototype model you probably came up against some problems and challenges which impacted on the final product. Maybe you realized that the product didn't look as you had imagined it, perhaps the scale, colors and form weren't right. Maybe the product was unsafe, or didn't function as you had initially intended.... It could have been that the design was too complicated or difficult to manufacture and some simplification was required. Whatever the reason you probably decided to alter the design and make some changes to improve it.
In Criterion C strand 4 you must outline what these changes were first. To do this it is a good idea to have a visual, maybe a graphic (screenshot of a fusion model, a sketch or a photograph) to show the change that was made. It is also a good idea to have a BEFORE and AFTER picture to show the initial design compared to the final prototype. Then use text to describe the change that was made, if you are MYP 4 or 5 make sure you include sufficient detail when describing the change.
Now you must justify why you made this change. In other words you must GIVE VALID REASONS OR EVIDENCE to show why you made these changes.
The more detailed the better.