1. Shut down the Apollo 8/8p and computer.
2. Disconnect any cables connected to the Apollo 8/8p except the power cable (this includes disconnecting the Thunderbolt cable). If there are any monitor speakers or headphones connected to the Apollo 8/8p, make sure that they are either disconnected or powered off before continuing.
3. While the Apollo 8/8p is powered off press the following buttons - Preamp knob, Phase and High Pass buttons (see picture below for reference).
4. With the buttons pressed, turn on the power switch to the Apollo 8/8p - hold down the knob/button combo for approximately 10 seconds until the LED lights on the front panel of the Apollo 8/8p stop flashing.
5. Once the LED lights stop flashing you can release all three buttons, shut down the Apollo 8/8p once more, and reconnect all cabling. Reboot the system like normal (it is recommended to always turn the Apollo 8/8p on before turning the computer on).