All property data is correct & Zestimate is too low. How do I get the Zestimate higher?

Well you are going to get lot's of advice telling you that zillow is just a starting point, with zillions of bits of information, etc but don't believe it.  [deleted by Zillow moderator. Please refer to our Good Neighbor Policy]. Basically it is a 'tool' to attract people to this website to generate revenue. Just like bees around a honeypot, and it works as they just got you here.

If everything is correct, there is nothing you can do to change the Zestimate.   The Zestimate as a "starting point" implies it is not taking into account upgrades - there is no consistent way to get upgrades from public data so Zillow chooses an approach to level the playing field.

but, there are things you can do:

a.  You can study the Zestimate range and emphasize the range.
b.  The owner or agent can put in data into the Zestimate "owner's comments" on the home details page. 
c.  Have you watched the Zestimate for Realtor's video to help know how to explain the Zestimate to your clients and potential purchasers?
d.  Have you studied all the AVMs from the dozen or more other sites that generate them to see how they compare?

A statistical model is not as good as in person visit by those who know the area.   There are some people who are very upset by this, but as an agent there are so many learning to manage the impact is a skill that is critical.