Verify that the printer is turned on.

Start PrusaSlicer and navigate to the top menu Configuration -> Flash printer firmware

Select the appropriate Bear calibration firmware *.hex file using the Browse button.

Verify that the printer is detected in the "Serial port:" window. If not, check that the USB cable is firmly connected to both the printer and the computer. select "Rescan"

Click the button Flash! to flash the Bear calibration firmware.

Never turn off the printer or disconnect the USB cable while flashing the firmware!

Wait until you get a Flashing succeeded! message.

In case of issues while flashing the firmware read the Prusa troubleshooting article.