Like John said, do a search and there's lots of info on how to install a new OS.
Make sure to follow the directions carefully.
And if you are referring to getting the "autodim off" option back, make sure the OS that you install will do that.
Bonus Comment: From all the posts I have seen over time, if you screw up the install I believe you can start all over again. In other words, it's near impossible to fry the system. This is because when performing an install you are basically wiping the chip clean and putting the OS back on to the chip.
Bonus Comment: John Clark is the resident expert on installing an OS [not sure how that happened]. Look for his posts [and his avatar, a plane going down in flames] and do what he says.