Property damage due to seepage from owner at top floor

You report the matter to your association and request them to interfere.

If there's no response or cooperation from RWA then you make a complaint with the local civic body about it or approach cooperative court after serving legal notice to all concerned in this regard 

You cannot demand the upper floor neighbour to sell his property to you.

Yes, you can sell legal remedy. However, you cannot compel him to sell the property to you.

Yes.  You shall discuss the issue with the committee, and if the RWA committee does not respond, report to the Registrar of Societies and send a legal notice to the RWA.  But you shall not compel the top-floor owner.

Well analysed, opined and advised by learned experts, I endorse it.

After availing social platform, if you are unable to convience the owner of top floor you may file a suit for mandatory & permanent injunction and claim for compensation. 

Leakage from the upper floor to lower floor is a tricky problem with no easy solution. If yours is a co-operative housing society you can try co-operative court. If you have an association registered under the Societies Registration Act the Registrar of Societies can do nothing. If the upper floor fellow refuses to sell his flat, you sell your flat and go elsewhere where is there is no leakge from upper floor. A goonda, who may be ready to deal with the upper floor may buy your flat, at a reduced price, of course.

The Cooperative Societies are governed by State Cooperative Societies Act / Rules and Bye-Laws adopted  by the Society (which are  approved by the State Registrar Cooperative Societies) and membership in CS is mandatory.

An association is a voluntary organisation registered under the provisions of Societies Registration Act, 1860 for the welfare of the members of association, which has no tooth to initiate coercive action against a member (if required). 

Accordingly there is basic difference between a Cooperative Society and an Association.