well, it's a delicate matter. I basically have an empty tub of the same size. I un-hook and pick up the lid and raise out of the res container and out of the grow cabinet and set into the clean empty container. I then remove the res. and empty and drain it all. I wipe it out real good with a bunch of paper towels and clean the air bubbler real good of any slime feel. Make sure it's all good and clean and dry and then put my water in that I have had been setting at room temp (or above) into the res. and add the correct ppm of nute and stuff then ph balance. Then I reassemble everything and carefully place roots and plants back into res and into closet, hook everything back up and then back to game on.
I've only changed my Res about 4 times this grow. It becomes more difficult with the older the plants get as there is more roots and lots more plant to be cautious of.
I haven't been able to tell of any shock from water at room temp or above.
I can't really tell ya how often. It just feels right or about time? Most places advise about every 2 weeks? Nah, maybe about every 2-4 weeks. You'll know after a few times. Your tank starts to get a real aquarium type smell, and You added fertz about 2 times or so in since your last change, That's when I change. You need to change it every 2 weeks if you maintain like 70 degrees constant, but mine drops when the temp drops a bit and when temps are down for a while I won't need to change res. for a longer period of time.
Hope this helps...