Drupal update the user email and name using user id UID

This is a small function used to update the user email
 /*
@param $uid Is the user id of the particular user
@param $email New email address to change to the particular user UID
*/

function UpdateUserEmail($uid, $email){
  // load user object
$existingUser = user_load($uid);

// create an array of properties to update you can add what ever user property you want to edit in this array.
$edit = array(
  'name' => $email,
  'mail' => $email,
  'init' => $email,
);

// save existing user
  user_save(
    (object) array('uid' => $existingUser->uid),
    $edit);
// load user object
}

Refference links

https://www.drupal.org/node/1969192
 

How to validate and submit a form using AJAX in Drupal ?

Drupal convert your form in to ajax 

Below is the example of ajaxifying your drupal form. all you have to do is write the call back

wrapper Form api will ajaxify your form.  

function test_form($form, &$fstate) {
  $form["wrapper"] = array("#markup" => "<div id='test-ajax'></div>");
  $form["name"] = array("#type" => "textfield", "#required" => true, "#title" => "Name");
  $form["submit"] = array(
    "#type" => "submit", 
    "#value" => "Send", 
    "#ajax" => array(
    "callback" => "test_form_callback", 
    "wrapper" => "test-ajax",
    "effect" => "fade"));
  return $form;
}

function test_form_callback($form, &$fstate) {
  return "<div id='test-ajax'>Wrapper Div</div>";
}

function form_validate($form, &$fstate) {
  form_set_error("name", "your  error to display.");
}

Reference Links: 

http://drupal.stackexchange.com/questions/12289/how-to-validate-and-submit-a-form-using-ajax

http://drupal.stackexchange.com/questions/6327/how-to-ajaxify-webform-submit-in-drupal-7

https://www.drupal.org/node/2081275

https://gist.github.com/pascalduez/1517513

Filter out specific errors during validation or modify any error message

This is one of the way to alter the error message in any of the forms.

In my example I have added a custom validate to validate the user login functinality.

1. filter out validation

<?php/**
 * Custom Form Validation.
 * Removes all form validation errors caused by a 'foo][bar' form element.
 */function my_module_form_validate($form, &$form_state) {
  $errors = form_get_errors();
  if ($errors) {
    // Clear errors.
    form_clear_error();
    // Clear error messages.
    $error_messages = drupal_get_messages('error');
    // Initialize an array where removed error messages are stored.
    $removed_messages = array();

    // Remove all errors originated by the 'foo][bar' element.
    foreach ($errors as $name => $error_message) {
      if ($name == 'foo][bar') {
        $removed_messages[] = $error_message;
        unset($errors[$name]);
      }
    }

    // Reinstate remaining errors.
    foreach ($errors as $name => $error) {
      form_set_error($name, $error);
      // form_set_error() calls drupal_set_message(), so we have to filter out
      // these from the error messages as well.
      $removed_messages[] = $error;
    }

    // Reinstate remaining error messages (which, at this point, are messages that
    // were originated outside of the validation process).
    foreach (array_diff($error_messages['error'], $removed_messages) as $message) {
      drupal_set_message($message, 'error');      
    }
  }
}?>

2. To modify the error message : 

 in the above code replace this code

 // Remove all errors originated by the 'foo][bar' element.
    foreach ($errors as $name => $error_message) {
      if ($name == 'foo][bar') {
        $removed_messages[] = $error_message;
        unset($errors[$name]);
      }
    }

with this one 

 // Remove all errors originated by the 'foo][bar' element. used this in user form alter custom validation
    foreach ($errors as $name => $error_message) {
      if ($name == 'name') {

       $errors[$name] = "<p>We can't find a user with that email.";
        $removed_messages[] = $error_message;
       // unset($errors[$name]);
      }
    } 

 This will replace the default error message "Sorry, Unrecognized username and password with the above text

Reference link : https://api.drupal.org/comment/28464#comment-28464  

Drupal - How to create or apply patches

This Url has a definitive guide for applying patch in the  drupal.org

https://www.drupal.org/project/drupal/git-instructions