Update custom user fields programatically how can I update custom user fields programmatically?

I've tried this but don't know how to get Id because I have to execute this code automatically on page load ..

$existingUser = user_load($uid);
$edit = (array) $existingUser; 
$edit['field_company_name']['und'][0]['value'] = 'Ciklum';
user_save($existingUser, $edit);
It should be something like this:

$existingUser = user_load($uid);
$existingUser -&gt; field_company_name['und'][0]['value'] = "Ciklum";
user_save($existingUser);