How to modify a form field? i want to modify a Select option field of webform with hook_form_FORM_ID_alter(), but when I do something like: 

$form['submitted']['dia'] = array(
'#type' =&gt; 'select',
'#title' =&gt; 'Dia',
'#options' =&gt; $options_first,
'#default_value' =&gt; $selected,
'#ajax' =&gt; array(
  'callback' =&gt; 'ajax_example_dependent_dropdown_callback',
  'wrapper' =&gt; 'dropdown-second-replace',
),
);


all the other values are replaced and also the weight.

I only need to add 'options', 'default_value', and 'ajax'. If I remove other properties like, 'type' the field is not displayed, and the other properties like weight is removed if I do what is in the code. How to modify these 3 attributes?
When you are using this syntax:

$form['submitted']['dia'] = array(


you create a whole new array, with only the values you provided. Old array that was there is simply discarded. If you only want to add or modify 'options', 'default_value', and 'ajax', you need to access them directly:

$form['submitted']['dia']['options'] = $options_first;
$form['submitted']['dia']['default_value'] = $selected;
$form['submitted']['dia']['ajax'] = array(
  'callback' =&gt; 'ajax_example_dependent_dropdown_callback',
  'wrapper' =&gt; 'dropdown-second-replace',
);