My niece gave me her old Iphone 6s Plus but has Completely forgotten her Icloud passcode. What do I do?

You niece needs to recover her AppleID and/or passcode so

she can remove the Activation Lock.

If she purchased a new iPhone and is using the same account (she should be since that's where all of her App Store purchases will be), then the phone she gave you will have the exact same ID and password. Use that one.