Delayed Possession by builder - what can i do to get relief

I was book the Flat in thane on 26/09/2014 as paid token amount & agreement is done on 08/12/2014 that they (Builders/Promoters) have clearly mentioned, they are give possession on or before Oct' 2016. But still I'm not get possession and now they said to me they are give a possession on Sept.2017. Also, one clause mentioned in the agreement If the promoters fails or neglect to give possession to the purchaser by the aforesaid date or dates prescribed in clause then promoters shall forthwith refund to the purchaser the amount already received by him in respect of the premises with simple interest @9% P.A. from the date of promoters the sum till date the entire amount and interest thereon is refunded by the promoters to the purchaser. What I can do?