There is another thread about this "who has flushed a sealed toyota transmission"
Basically you can do it but it requires a process. You have to go thru a step by step process to make sure the fluid is a certain temp when you are checking to fluid level. I just done mine yesterday with a BG trans machine that hooks up to the lines. It basically was a fluid exchange. I used amsoil atl which meets toyota ws fluid specs.
Unless you are prepared to go thru a tedious process I would let someone with a fluid exchange machine do it. My toyota dealer told me $159.....call a couple others and see who's cheapest. If you can get it for $160 or less I would let them do it cause the fluid is expensive even if you tried it.