There is a Smart Object on the mockup to help you to adjust your design. 

Double click on the layers with the smart objects (called place your design here).
Inside the smart object, Just drag and drop it your image. Then save the file and it already appears in the original file.

here are layers of color to color the objects.

You will also find solid color layers, to change colors of the objects.

The high resolution of this mockup allows (300 dpi and more than 2000x2000) it to be used for commercial purposes. Enjoy the premium quality!