In Malaysia, there's no law that protects owners for this kind of situation. You can try to consult a lawyer for this but most of them would ask you to settle the problem yourself rather than paying the expensive lawyer fees.

Imagine if your own unit leaks and the lower unit suffers, would you pay for the fix? It's already very good that if you can get your upper unit to cooperate and let you do the fix inside their house, don't even hope they will pay or share the cost with you.