- copy NEW/CURRENT firmware to the card - insert into camera.
- launch Update Firmware from Canon menu, click OK
- open the CARD door ASAP, but don't remove the card or it won't work and will give an error message
- QUICKLY insert a SECOND card with the older firmware on it into the other card slot
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- If done correctly, the above menu should pop up. Select the older firmware and install
I'm not sure if this method works on the R5 or any other camera.