Press the BOOTSEL button and hold it while you connect the other end of the micro USB cable to your computer:
Pro Micro
Short the rst pin to ground twice in quick succession, and the tool will detect the pro micro in bootloader mode
Note that when this is done, you only get around 8 seconds to reprogram the guitar, so you will need to be quick and its best to open the config tool first.
Clearing the config on a Pro Micro
Install Arduino IDE https://www.arduino.cc/en/software
From the file menu, load up the EEPROM clear example sketch, as shown below:
Go to file -> preferences -> additional boards manager -> URLs and put in: https://raw.githubusercontent.com/sparkfun/Arduino_Boards/master/IDE_Board_Manager/package_sparkfun_index.json
Go to tools -> board -> board manager
Search for "sparkfun" and install "SparkFun AVR Boards"
Set your Arduino type as SparkFun -> Arduino pro micro
Set the processor to the voltage of your arduino
With the arduino plugged in, short the rst pin to ground twice in quick succession
Quickly pick the serial port that shows up
Hit upload
Quickly short rst to ground again twice
Note: It is important to hit upload first and then short twice, as the Arduino only stays in it's programmable mode for 8 seconds