Changing the plugs on my 2001 requires you to remove the manifold and other parts. The dealer charges close to 400$ for 6 new plugs installed so the procedure is beyond me and probably you to. Good luck.
Step 1, find sparkplugs, they are at one end of the spark plug wires
Step 2, remove spark plug wires
Step 3, remove spark plugs (this is the part that is near impossible, I'm wondering if it's not made easier by loosening the motor mount and allowing the engine to rotate forward, that's what is suggested on some GM vans with comparable engine compartment constraints, I may try it once my plugs get so bad I feel like it's costing me more than $600 in gas)
Step 4, install new spark plug (this is just as hard as getting the plugs out, but in opposite order, be extra careful not to cross thread the plugs or you'll be hosed)
Step 5, re-attach spark plug wires
Step 6, re-install all the stuff you had to remove to get as far as step 3.
I did it on my 99' Sienna without too much trouble. Not sure if the engine design is the same or not. I had the manual which advised removing this and that but I tried it without removing much. I don't recall if I removed the plastic cowling that holds the windshield wipers, etc. or if I did that with even that in place.
The front 3 were easy as all I had to do was remove the coils or whatever those ignition things are called (sorry I don't have the manual in front of me) and then remove the 3 spark plugs. For the back, I got my wrench with the right sized extension (6" maybe?) and a spark plug socket and reached into the back (after removing the wires of course). I was able to remove the plug with the socket wrench. The spark plug socket kept the plug from falling out. Yes, it was awkward to get to but not impossible. Probably spent 30 minutes on the 3 in the back. No, my arm is not made out of rubber.........
Yes, the front is very easy but the back is a big pain. Your really have to crawl around the engine to get them. It took me three hours to do the back. Yes you are made of rubber to do them in 30 minutes.
This is where I found I am able to to things the dealer can't/won't. I have a 2000 Sienna, started missing real bad. Well, the first thing is to replace the plugs, they are not so bad, the 3 in back require a bit of patience. The missing started again. I took it to the dealer. They didn't charge $400, they charged $200. They said I put the wrong plugs in. It started missing again. I brought it to another, they said I had a cracked plug, cost me $100. Started missing again. I bought a OBD tester for a hundred, found the code, #5 misfire, I replaced the wires. Why wasn't the hi tech shops able to find a shorted out plug wire? I agree, engine cool, remove the plastic cowling, put your plastic arms on.