How to disable featured images from auto crop

Hello i really need help. How to fix or disable auto crop function in this theme? I’ve been looking for tutorial (example : https://help.magniumthemes.com/hc/en-us/articles/115002996813-How-to-disable-featured-images-auto-crop-in-WordPress) but when i take a look in the funtions.php i can’t find any like that. When i set any of my post on my blog with vertical featured images, all of the featured images became horizontal. I want to disable the auto resize/crop so the featured images wouldn’t cropped to horizontal. I’ll wait for any tutorial, thanks before.