You can follow these steps to change the lock pin on the phone.
Unlock a phone or tablet with Find My Mobile
In order to use this feature, Remote unlock needs to have been activated on the locked device.
Go to the Find My Mobile website.
First, navigate to the Find My Mobile website and log in with the same Samsung account used on your locked device. Use the menu on the left side of the screen to choose your device from the list.
It may take a few seconds for Find My Mobile to locate your device. The device must be powered on in order to be located.
Unlock your device and reset the security.
Once your device has been located, click Unlock, and then click Unlock again. Enter your Samsung account password, and then click Next to finish. Doing this will reset the Lock screen security type to swipe only.
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