Modify a 3D Model

I've attached a 3D model for an airport. WE NEED TO MODIFY TERRAIN SO WE CAN PLACE AN AIRPORT ON A FLAT SURFACE IN THE MIDDLE We need a large area in the middle of the terrain to be flattened out so we can add a large airport model there. It should be slightly raised up like its on a big hill but you must make sure it has a completely flat surface so the runway can be placed on it properly at a perfect angle. - Do not change the scale, rotation or position of the OBJ. - Do not change any of the vertices on the edges, this terrain piece fits into a larger scene and must not be changed in any way. - The hill that you create / modify should still look as realistic as possible. - You can change the vertices in the middle sections as much as you want as long as the upper surface is flat and has ample room for us to place a large model of a runway / airport.