Enter the amount you would like to exchange. In the second drop-down list select BUSD. You will be shown the estimated value that you receive after the exchange.
BUSD will be sent to this address. Be careful and make sure you use the correct Binance Usd address.
If everything is correct click the Exchange button. You will be shown deposit Bitcoin address. You need to transfer to this address the necessary amount of BTC to start the exchange.