To downgrade linux-firmware
 

How do we do downgrade linux-firmware as brightness setting got ruined: giving no effect at all even if the GUI, ie. mouse click, is responded in slider bar to go left to right
(it's suspected after update the firmware).

Please help solve out!

Try the downgrade package/software. It's in the AUR.
Or just find a lower firmware version and install it.
However, I'm not at all convinced that will actually help, and not make matters worse.
Show us some proof that it's really the firmware causing problems.