Scroll down to find your model (mine: Original Prusa I3 Mk3)
click download in Firmware column
a zip file will be downloaded
something like: prusa3d_fw_3_11_0_MK3.zip
Move the download to desired location, on my system:
.../Tech/3Dprinting/Prusa/software/...
create new directory there named: prusa3d_fw_3_11_0_MK3
extract the zip file.
7 pdf's will be created, each the same in different languages.
Delete the ones you don't want. Keep "..._EN.pdf"
The English pdf you kept will be instructions for proceeding.
Download FW to printer:
Connect USB cable between your computer and the printer.
Turn on the printer.
Open PrusaSlicer
Configuration (top menu bar) > Flash printer firmware
If your printer is not listed in the "serial port" field, press ReScan
Click browse icon at far right of the Firmware image field, and navigate to your downloaded and unzipped firmware folder
select: prusa3d_fw_MK3_3_9_1...hex, then Open
click Flash!
when "Flashing succeeded," click Close
disconnect USB cable
LCD Menu - Support should show new version of firmware