While updating your existing records, then validations are fire. if the rule is true as given condition then the records are not update which are effected by the validation.
@Nagarjuna Reddy is correct. You can export the Salesforce IDs of all the records, run an update and load the IDs onto themselves (with Apex Dataloader, or Dataloader.io), and the validation rule will fire on all of them. You will get errors for those that do not pass validation. You can then delete or update the records that have errors.
Use dataloader or import wizard to update the exhisting records and if the records passed the validation rule will be saved and others will not..
Cheers!
Nag Arjun