See the operation manual Section 7
I see the firmware installation in section 2.32 of the Assembly manual.
To me it seems clearly written, can even be printed out and each step ticked.
I always prefer good written instructions. Even if a good video is found it never goes at the speed you do, pausing, going back many times is a real pain.
Note the way to force bootloader mode. A loop of wire will usually be good enough. Make sure it is secure and not touching anything before applying power.
Alan
