To sell Bitcoin instantly on CoinDCX, go to prices and click on Bitcoin. It is here that you can directly press the sell button or click on the View Price Chart option to see how Bitcoin has been performing over the last couple of days/months.
Click on the Sell button and based on the current market price, select the amount of Bitcoin that you would like to sell from your existing portfolio. Once that is done, click on Sell BTC and your order will get executed instantly.
You can now view your remaining Bitcoins in your Investments and all your past orders in My Orders.