Most have answered correctly, that it depends upon whether you do it or you'll have someone else do it---becuase it depends upon the mess.
If you don't want to get dirty, then most places nowadays use flush machine. and it's expensive. From $120 up to $150. But as others say, it isn't any better, it's just less messy, the mechanics don't have to get all oily.
If you want to do it, then you just have to drain it out, replace plug and add the EXACT CORRECT amt of ATF.
The hardest part is to make sure that you've got the proper tool to unscrew the tranny plug. I'm not sure on modern Toyotas but my older 1991 uses a 17mm hex lever, and I have to use a small pipe over it to get enough leverage to turn the screw. So it's completely make-shift tool.
But otherwise, you should have car up on ramps. And have a good sized bucket or tray to drain old fluid into. You want to run engine to warm it up so fluid will be hot and drain easier. But when hot it also burns YOU. So you need to put couple sets of rubber gloves on. Draining ATF is actually much easier than motor oil, because Dexron hardly gets thick like motor oil does. So with very little turn of the drain plug, the old Dexron starts to leak out. But eventually, you want to unscrew the plug completely and let it all drain out.
New ATF is completely clear. Old is cloudy or dirty looking.
The other MOST IMPORTANT PART is to install the EXACT CORRECT amount of ATF. Motor oil is forgiving if you put in too much, but ATF is NOT. You cannot have too much in tranny, because otherwise, the fluid will bubble and then tranny loses ability to work. Last summer, travelling out of Tennessee, I had changed ATF week before and thot I put in correct amt, but got about pint too much. AFter driving 2 hrs, car suddenly loses power, engine running fine, but just won't drive car. Found out too much ATF, so had to drain some on edge of road into grass, and it ran fine afterwards.
(ATF is only a couple qts, not a gallon like motor oil. In fact, the amt between the full and need-to-add line is just a pint!! So it's very precise.)
So you need to research to find out the capacity of ATF in your car.
And then just make sure you replace drain plug before you add the new fluid. I didn't one time, and saw all the brand new fluid go right thru onto the ground!!
Stupid mistake, but a valid one to remember. And make drain plug tight, but don't close it up with all your strength. You've got to be able to remove plug sometime later. So make it tighter than snug, but not too much.
But I would use just plain Dexron. There's kinds that talk about old trannies that have high mileage on them. Don't worry about it. Reg Dexron is fine. There's up to about #4 I think, but #3 or 4 will work perfectly well.
Personally, I think it takes a while for Dexron to get broken in after first couple thousand miles after new.