As Megan says, you can update the information on Zillow. However (because I post a lot on Zillow, too), Zillow cautions that changes you make can take a while to be reflected in any new zestimate.
Beyond that, though, I don't know why anyone cares. Half the questions on Zillow seem to be about owners disagreeing with the zestimates. So? Realtors don't use Zillow to determine values. And Realtors will tell their clients that those numbers aren't reliable. And, frankly, if you're selling a property and some buyer rejects it because the zestimate is low, then you may be dealing with someone you'd rather avoid.