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.
Click the edit button to expand the editor, you will see an option to delete the current phone image.
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
Positioning: Absolute
Top: 200%
Left: 50%
Flexing: Constrain
See how it will look like after uploading a new image.
Trust this helps.