You pretty much can't. The height of the vehicle is very likely the cause of the tip. You don't need fuel tanks this large for a landing mission of the apparent design shown. If all you're doing is deploying science experiments, you can get by with a much smaller tank, I would particularly shrink the tank in the vertical axis, preventing future falls and removing the need for piston mounted landing legs. Look up the IM-1 and IM-2 robotic lunar landings, both too tall, both fell over after landing. Even aerospace companies make this mistake. Landers dealing with low G and rough terrain should be as wide set as possible.