how do unloc a phone that dont now the old owner

You can not do that.

If you do not have the Passcode for the phone, it must be restored to the factory settings. Everything will be deleted. If Find My iPhone is activated on the phone, then after the restore it will be Activation Locked, i.e. password for the previous user's Apple ID must be entered.

If you don't have that code, the iPhone is virtually useless.

You can read how to restore the iPhone if you do not have the Passcode or have forgotten it:

If you forgot your iPhone passcode - Apple Support

You can read more about Activation Lock here:

How to remove Activation Lock - Apple Support

if the phone is iCloud locked there is no way around this lock. it is locked out on a hardware level so even restoring the phone will not unlock it. the only way for you to unlock is to contact the previous owner and either get the credentials or have them remove the phone from their iCloud account. this is a theft deterrent first and foremost. if thieves know a stollen iPhone is useless there is little or no incentive to steal one.