How to update post's featured image in front-end

Question

I created a new template page with code to allow front-end posting. Everything works great, with tags, post meta and image uploading (as featured image).

I have seen many codes about front-end post editing, but even though I tried to figure it out I can not successfully make it to work.

I took as example this How can I edit a post from the frontend? and placed it in single.php with no luck. I guess the right way to have it, is to have an EDIT link below the post and there will be appear a new php template that contains the edit form when it is clicked. Is this correct?

Secondly, how can I update the featured image I added from the front-end ?

Thank you for your help.