How to change post featured image using a custom field of category?

I have added an image custom field for categories, in which I assign an image for every category. Something like this:

I have a custom post type, and for each post I publish in this post type, I assign one category. I need each post I publish to change its featured image, according to the category assigned to it, with the image that has been added to the custom field.

Any help?

Edit:

I found this topic about the same question:

Post featured image from category custom field image.

There’s an explanation on how to do it. Hopefully, someone can help me with the code since I’m not skilled enough to be able to do it.