Hello and welcome to PSG.
This is a simple Clipping mask technique.
All you need to do is to create a template of the area you would like to replace, in this case it's the phone case. To do this you will use the Pen Tool.
Then it's just a matter of adding images on the layers above the template and clipping them to the template layer.
Template..
New image (clipped to template)...
Add another image on top of the last image and clip it as well.
(since it's on top it will be visible, so no need to turn off the other layer, but you can if you want)
Here is the layers panel.....
Also notice I added a shadow layer at the very top. I did this by Cmd/Cntrl + Clicking the template layer to make a selection and then I used the gradient tool create the gradient. I set this layers blending mode to multiply and lowered it''s opacity.
Here are many videos on how to create clipping masks:
https://www.youtube.com/results?search_query=photoshop+clipping+mask+tutorial