You can reinstall the default seesaw Firmware using the UPDI programmer, megaTinyCore and the Adafruit_seesawPeripheral library.

Install the Libraries

Install the Adafruit_seesawPeripheral library, Adafruit_BusIO library and the Adafruit_Seesaw library by cloning the repository and adding the folder to your Arduino libraries folder on your computer.

Upload the seesaw Firmware

In the Arduino IDE, go to File - Examples - Adafruit seesaw Peripheral.

The example_pid#### sketches contain the default seesaw firmware for each seesaw breakout. The pid#### portion of the name corresponds to the Adafruit product ID of the board. For example, to upload the firmware to the ATtiny816 breakout, you would select example_pid5681.

After selecting the corresponding firmware sketch, upload it to your board using the UPDI programmer. You can confirm that the seesaw firmware was successfully uploaded by connecting the ATtiny breakout to a board via I2C and performing an I2C scan. The address 0x49 should appear.