how can I change the clock frequency in DUE. Its default is 4 mhz. I want it at 16mhz
OK, so you're actually just using the Due as a USB to serial adapter. For that purpose you should connect sensor RX to Due's Rx0 and sensor Tx to Due's Tx0. I know that seems wrong at a first glance but the pin labels are relative to the SAM microcontroller pins but you're not using the microcontroller, you're using the ATmega16U2 USB-serial chip, which is connected to 0 and 1 opposite to the labels on the pins (because it's connected to the SAM microcontroller RX-TX, TX-RX). So when you make the RX-RX, TX-TX connections between the due and the sensor you're actually doing Rx-Tx, Tx-Rx with the ATmega16U2.