You need to have Photoshop CS4+ version.
Each file has different mockups – switch between them in the folders.
Find the “Change this” layer, click twice on the icon and a new window will open. Insert your design in the window, close it and save.
You can change the devices’ color in the Changeable Color folder. To do this, click twice on the “Change Color” icon and choose the needed color.
Change the Opacity parameter for Reflection and Fill Layers, if you need to set up the contrast.