How to change the label of username field on all forms/pages that it occurs?
Add the following code at the bottom of the settings.php file:

 $conf['locale_custom_strings_en'][''] = array(
   'username'      =&gt; 'nick-name',
 );


As alternative, you can use the String Overrides module.


  Provides a quick and easy way to replace any text on the site.
  
  Features
  
  
  Easily replace anything that's passed through t()
  Locale support, allowing you to override strings in any language
  Ability to import/export *.po files, for easy migration from the Locale module
  Note that this is not a replacement to Locale as having thousands of overrides can cause more pain then benefit. Use this only if you need a few easy text changes.