1) Login to Android Device Manager from a computer.
https://accounts.google.com/signin/v2/identifier?service=androidconsole&passive=3600&continue=https%3A%2F%2Fwww.google.com%2Fandroid%2Ffind%3Fu%3D0%26sid%3D7Sam40&followup=https%3A%2F%2Fwww.google.com%2Fandroid%2Ffind%3Fu%3D0%26sid%3D7Sam40<mpl=findmydevice&flowName=GlifWebSignIn&flowEntry=ServiceLogin
2) Use the "Find my Device" function to find that phone.
3) there should be an option to set or reset a new passcode.