How to fetch the existing value of a form field in the form?
You can use #default_value attribute of the managed_file to show the existing value of the field.

You can get the value of fid from database and set value into #default_value.

Example:

$form['picture'] = array(
  '#title' =&gt; t('Image'),
  '#type' =&gt; 'managed_file',
  '#description' =&gt; '',
  '#default_value' =&gt; 1, //fid from database
  '#upload_location' =&gt; 'public://image_example_images/',
);