Retrieve Drupal user id from CiviCRM Contact id
As others have noted it's not clear what you're asking but based on your wording I'm going to assume you mean via code. You can use the api explorer at https://mysite.org/civicrm/api (replace "mysite" with your actual site) to explore what's available and give you some sample code. Here's how to get the drupal uid given the civi contact id.