Here is a basic tutorial a wrote a while ago as a response to an email question:
To do this, you will need the ESC, a power supply for the ESC, two male to male jumper wires, a computer, this software running (download BlHeliSuite, not BlHeliSuite32), and a device such as an Arduino to act as the interface between the ESC and your computer. I am writing this tutorial for an Arduino Uno board, as that is most common board and a very versatile tool to have.
Connect the Arduino to your computer via USB and start the BlHeliSuite software.
Go to the “Make Interfaces” tab. Select your Arduino board (Uno w/ATmega328) as well as the appropriate COM port with the Arduino, and click “Arduino BLHeli Bootloader” (option C for SiLabs microcontrollers). This programs your Arduino with the firmware necessary to allow it to communicate with the ESC. You can leave the baud rate as default, but take note of it.
Next, make sure the ESC is disconnected from the thruster/and motor and the 3 motor wires are no making contact with each other. Using the male to male jumper wires, connect the signal ground (black) to ground (GND) on the Arduino, any open ground will do. Connect the signal PWM line (white) to digital pin 11 on the Arduino.
Go to the “ESC Setup” tab in the software. Click the “Select ATMEL/SILABS Interface” menu at the top, and click “SILABS BlHeli Bootloader (USB/Com)” (option C for SiLabs microcontrollers).
Power the ESC with your power supply of choice, and select the COM port with the Arduino and the baud rate noted previously. Click “Read Setup”. The interface should load the ESC firmware and present you with a number of option to change.