Updating fields in existing records from csv file
Its possible to update the contact using the same csv if you have unique identifier for each row and the same is stored in CiviCRM database either as CiviCRM Contact ID or contact external identifier for each contact. 

During import select 'Update' for duplicate contacts and then in mapping make sure you use either external identifier or contact id. If this 2 fields are used during import than the contact's are directly updated into the system without performing any dedupe checks like firstname-lastname or email which is again unique way of avoiding duplicate contact into system. 



If you have stored custom unique key field for each record in contact's custom field then probably you will need to run a query which will update the external identifier for a contact and make it easy for you to import again using same sheet.

The sql would be something like:

UPDATE civicrm_contact cc
INNER JOIN civicrm_value_custom_table cv ON cv.entity_id = cc.id
SET cc.external_identifier = cv.custom_unique_id_field;



  Note: Backup your database before performing contact import or running
  any  Insert/Update/Delete sql.


HTH 

Pradeep