Recommended 2 year interval, I've extend to 3, due to the very high quality of our water, lacking nearly all mineral content, and being very nearly distilled. Changes too often will introduce excessive corrosive agents and other contaminants, which after a period of reactivity (and damage) will dissipate, leaving the coolant ideally chemically "dead".
The changing procedure is covered on pages HT-6 and -7 of volume 2 of the repair manual. I find the description deplorable, and to this day wonder what's intended. I believe it's an attempt at purging all the air from the system in one refilling operation. Let me say emphatically, that this is simply IMPOSSIBLE, and be prepared to top-up the coolant after each operation, much more at first and then lesser with time until the level becomes stable.
Other informed perceptions on this most welcome & thanks ! ...