To sell Zilliqa, simply go to the exchange where you hold it, select the Zilliqa pair you bought, choose the quantity of ZIL you want to liquidate and hit the 'sell' button. Your ZIL will revert to the quote currency, so USDT if you bought ZIL/USDT. From there you can buy another crypto with your USDT or convert it to your preferred fiat currency such as USD.