how to upload an image on ACF with update_field on wordpress

I had the same problem, you almost had it right, which helped me as well.

looked at ACF and the update_field accepts attachment ID as oppose to the the, pid or url

You can replace the return array:

return array(
    'pid' =>$pid,
    'url' =>$file['url']
);

With the following array:

return array(
  'pid' =>$pid,
  'url' =>$file['url'],
  'file'=>$file,
  'attach_id'=>$attach_id
);

and then change

update_field('field_512e085a9372a',$att['url'],$watch);

with the following

update_field('field_512e085a9372a',$att['attach_id'],$watch);

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