Alright so that must mean it’s your faces. In edit mode and in face selection mode, select all the top faces and then S to scale, Z to lock the scaling to the Z axis and zero (0) to scale all those faces straight.
Is that doesn’t work, then it might be that you face is too broad and you have too big of an Ngon there, so you need to cut some new faces. You could do that with the knife tool or by creating extra vertices by subdividing and joining them with the J key.
