Closing The Deal Of Sell Of A Flat

I have made a contract to sell my flat situated in Greater Noida (west). The buyer had to pay full amount with 45 days with grace period of 15 days as per the sale deed. But the buyer has not paid the full amount within the contract period. However I have given him the physical possession but registry of flat is not done till date. Now since the contract has been expired and buyer was failed to make full payment within the contract period and rate has been increased in the market, therefore I want a fresh deal at current price. But buyer is not responding any more. Now I want to close this deal. I want a legal notice to be given to the buyer for vacating my flat. KIndly help me, how to get legal notice be served to the buyer.