Changing form field title within hook_preprocess_form() does not work I have customize drupal user registration form which I want to change some field titles.

I implemented hook_preprocess_form() within theme and changed titles of some form fields, but it didn't work.

While debugging I found that it has '#children' item already there with rendered HTML inside it. Then I tried to change that value by using drupal_render_children(), but that also doesn't work.

What is the right way?
Use hook_form_alter() to change the form before it gets rendered.