Changing position of the header in a view
In your custom theme check if there is a templates/views-view.html.twig file 

If not you you can get a copy from core/themes/stable/templates/views/views-view.html.twig and modify the position of header section to be below exposed 

the content should look similar to the following after your change

...
{{ exposed }}
{% if header %}
  &lt;header&gt;
    {{ header }}
  &lt;/header&gt;
{% endif %}
...


This will customize all views in your current installation

If you want to only modify a specific view you can name the file templates/views-view--VIEWNAME.html.twig

Here is more info on the template naming convention