1) To crop a setup, first choose a space and place an artwork. 

You can read How to create a mockup / setup to learn how to create your first setup. 

2) Click or tap on the three dots on the bottom right-hand corner and select Crop. 
Remember, you will need to have an artwork placed in the space first, otherwise you'll find the tool disabled. 

3) Once you've selected crop, you'll see a selection of what is to be cropped. 

4) You can now adjust the selection by pulling it from the corners and dragging it. 

You can choose standard sizes for social media and common aspect ratios, or select your own sizes for projects and promotion.

5) Hit the save button on the bottom right hand corner and continue editing.