Just seeing this sorry! I wouldn’t flip it over. With the table upright, start turning the turnbuckle one way.
If it helps level it, keep going.
If it doesn’t help, turn the turnbuckle the other way until the table is level to your liking.
If you are asking how to turn a turnbuckle, just grasp it and continuously twist it in the plane of the cables one way or the other to tighten or loosen it.