def update(username, realm, password)
      raise DigestFileError, "Unable to update non-existent user #{username} in realm #{realm}" unless has_entry?(username, realm)
      ir = internal_record(username, realm)
      ir['entry'].password = password
      @lines[ir['line_index']] = ir['entry'].to_s
      dirty!
      return nil
    end
def update(username, realm, password)
      raise DigestFileError, "Unable to update non-existent user #{username} in realm #{realm}" unless has_entry?(username, realm)
      ir = internal_record(username, realm)
      ir['entry'].password = password
      @lines[ir['line_index']] = ir['entry'].to_s
      dirty!
      return nil
    end
Public: Updates an existing username / relam entry with a new password

 username - the username of the entry
 realm    - the realm of the entry
 password - the password of the entry

 Examples

   digest_file.update("existinguser", "realm", "newpassword")
   digest_file.save!

 Returns nothing
 Raises DigestfileError if the username / realm is not found in the file