As per the instructions, if you'd like to reprogram the chip, you can setup an Arduino-as-ISP programming circuit either on a breadboard or by using a programming shield such as this one:
https://www.tindie.com/products/siktec/all-attiny-arduino-programmer-shield/
Recommended tutorial:
https://www.instructables.com/Program-an-ATtiny-with-Arduino/
To recompile the firmware, you will need Spence Konde's ATTinyCore if you want to continue using the tone() function as on a normal Arduino device.
Once you've got that installed, you'll want the upload configuration to look like this (choosing the appropriate port for your board and computer setup):
