1) Login to Joomla Administrator. 2) In the top menu, hover over Extensions and click Language Manager 3) If you haven't created any overrides yet, the list will be blank and you'll need to start adding them. 4) Click the Overrides Tab and then click New 3) Create a new constant and write the "Language Constant" as in en-GB.override.ini file. This content will override the Joomla25\language\overrides\en-GB.override.ini file content and the changes done by the admin will appear to user.

E.g. For managing the password Reset Subject from admin

Create the new Override using Language Constant "COM_USERS_EMAIL_PASSWORD_RESET_SUBJECT".
And at front end it will be used like this:
$subject = JText::sprintf('COM_USERS_EMAIL_PASSWORD_RESET_SUBJECT');