Forgotten Android Phone PIN

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I've been asked if I can get into a parent in laws phone (because it is my old phone) as they have forgotten the PIN.  I have tried what I thought their password is, but it is not correct. How can I get into the phone. From all accounts they have not been using it so if I need to factory reset that is fine, but I'd rather just be able to unlock it so I don't need to set it up again.

 

Any ideas?