Used MacBook Pro still tied to former owner's Apple ID

I inherited a MacBook Pro (Retina, 13-inch, Late 2013) which I upgraded to Mojave. When I go to perform individual updates the system reports that this machine is still associated with former owner's Apple ID. I need to change it to mine so that I can perform updates. I do not have password to prior owner's Apple ID. When I go to do updates the UserId is greyed out. Help please.