Update post thumbnail with value of ACF Image field

acf-update-post-thumbnail.php

<?php

function acf_update_post_thumbnail( $post_id ) {
    $thumbnail_id = get_post_meta( $post_id, 'acf_image_field' );
    set_post_thumbnail( $post_id, $thumbnail_id );
}

// run after ACF saves the $_POST['acf'] data
add_action( 'acf/save_post', 'acf_update_post_thumbnail', 20 );