In the Mobile App theme, the phone is an image itself. Therefore, there is not an option to edit or add a screenshot inside of the phone image. You will have to upload a new image as a whole.
What you can do though is upload a new image similar to the phone image as a whole.
To do that, all you need to do is delete the current phone image. To delete it, mouse over the current image, an edit button will show.
Once deleted, you can now upload a new phone image. Select the option to upload a new image.
Once uploaded make sure to set it to
See how it will look like after uploading a new image.
Trust this helps.