How to modify the existing field in a form? I wanted to modify the existing fields in a form. I tried using hook_form_FORM_ID_alter but I could only add new fields; I could not alter the properties of existing field. I wanted to add ajax properties to some existing fields. 

I wanted to alter the content add form which is created by Content Type. I wanted to add an Ajax property to the content creation form. I couldn't figure out a way to add this while creating the content type. Could anyone please help on how to do this? It's very urgent. 

This is the DPM for the field. 

field_mstyle (Array, 6 elements)

#type (String, 9 characters ) container
#attributes (Array, 1 element)
#weight (String, 2 characters ) -3
#tree (Boolean) TRUE
#language (String, 3 characters ) und
und (Array, 18 elements)
    #entity_type (String, 4 characters ) node
    #bundle (String, 12 characters ) pz_main_menu
    #field_name (String, 12 characters ) field_mstyle
    #language (String, 3 characters ) und
    #field_parents (Array, 0 elements)
    #columns (Array, 1 element)
    #title (String, 10 characters ) Menu Style
    #description (String, 0 characters )
    #required (Boolean) FALSE
    #delta (Integer) 0
    #type (String, 6 characters ) select
    #default_value (Array, 1 element)
    #multiple (Boolean) FALSE
    #options (Array, 4 elements)
    #value_key (String, 5 characters ) value
    #element_validate (Array, 1 element)
    #properties (Array, 4 elements)
    #after_build (Array, 1 element)
You'll usually find the elements for fields buried in the form and you'd alter them like so:

$form['field_my_field']['und'][0]['value']['#ajax'] = array(
  // ajax settings
);


I strongly recommend installing the Devel module and using the dpm() function to perform debugging. You can use it to print any variable to the standard message area, and it allows you to inspect arrays and objects.

You could easily find out where abouts in the $form array the fields you're looking for are by including a line like this in your form alter function:

dpm($form);