I can't advise on how to replace them (haven't done it myself yet), but I'll give you personal experience on what to replace them with -- OEM plugs only!
My car's PO used Bosch Platinum plugs and had them replaced at the same time as they had new wires, cap and rotor replaced (was my father so I know it was actually done). This was 20K miles ago. I just had to have *all* the same work done again because the Bosch plugs apparently have VERY little platinum on them, and as the platinum wore away into the material beneath it the gap increased significantly, to the point where the spark started looking for a path of lower resistence and found it. The new path was through the rubber boot on the wires and to the block. I had melted boots around a couple of the plugs and the whole deal (cap, rotor, plugs, wires) needed to be replaced after being installed for only 20K miles. The Bosch plugs were $2/ea the Denso plugs were $10/ea. Having the whole job redone was $600, which if the right plugs had been installed previously, could have been avoided.
My repair guy (been my guy for 12 years on a variety of foreign cars) told me that w/ Lexus', there's quite a few parts that it's just not wise to use other than OEM. This from a guy who has always been very above board w/ me and saved me tons of $$ over the years by suggesting non-OEM stuff in repairs where it didn't matter.