How to remove the featured imagem from the top of the post?

To resolve this issue you can log in to Dashboard > Theme > Customize > General Settings then add the following code to the “Header Script”:

 
.single-post .entry-thumbnail img {
display: none;
}

To display the Related post image, you can add the following code to the “Header Script”

<style>
.single .related-post .entry-thumbnail img {
    display: block;
}
</style>