How to remove the flat lands area in a minecraft world?

I have a map that I've been working on for a while and it has flat lands all around it.

I wanna remove it but I can't. If I re-create the world with the minecraft settings it gets rid of all my work.

Is there a method to do this? Can you just use world painter to remove the flat lands? If so then how?

Yes i think you could but i prefer World Edit
Its better, simplier and easier to use