Changing Header Cropping Tool Dimensions

Hello,
I am running the 2016 WordPress theme and I am using a child theme. I would like to change the dimensions of the cropping tool in the header image tool from 1200 x 280 to 1200 x 400
I have had a bit of play around but have not been able to get there. I have done it before and it worked (on the 2013 theme) but this time around I can’t do it, most likely due to lack of experience
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So i am hoping someone may be able to point me in the right direction of how to change the dimensions of the cropping tool for the header image so when I add a new image I can crop at a height of 400 px
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Thanks