1. download pocketinveditor pro
2. while download or before downloading make a creative world in mcpe , at spawn dig down and when you hit the last bedrock , exit mcpe
3. when its done downloading > open the app > click the map that you want to be flatten > go to edit terrain > then click user player coordinates > in width (x):500 height (y):1000 length (z): 500 > click set block > click browse > select the first one and then click ok >wait while reading terrain data is analyzing and when its done go back to mcpe > open the world > see if it works

p.s.
you can do it too in the surface of the map but if you want to build a super high building or skyscraper dig down