sudo nano /etc/apt/sources.list                                                    #Type this command in cmd, followed by 'enter' to open the file in nano
deb http://raspbian.raspberrypi.org/raspbian/ stretch main contrib non-free rpi    # paste this line using 'ctrl + shift + v'

# Use the '#' in front of previous existing lines to comment them out!
# press 'ctrl + o' to save the file!
# press 'ctrl + x' to exit nano!

sudo apt-get update                                                                # Once existed, continue with updating aptitude to make the changes take effect.