how to unlock a iphone if the dont have previos owner info

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Find My iPhone Activation Lock - Apple Support

How do I check for Activation Lock before I buy a used device?

When you buy an iPhone, iPad, iPod touch, or Apple Watch from someone other than Apple or an authorized Apple reseller, it is up to you to ensure that the device is erased and no longer linked to the previous owner’s account. Follow these steps before you buy the device:

1. Turn the device on and slide to unlock.
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. After choosing a language, choosing a country or region, and connecting to a network, the device will begin activation. If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, they can remove the device from their account by signing in to icloud.com/find. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.

You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.