Easiest way to do DIY though is check the level when warm (trans warm, not engine warm), drain, measure the amount that came out and top up with that same amount of fresh fluid.
Then you can either
1. Do it again in a week to minimise the content of "old fluid" (easy/safer way, but time consuming/more costly) or
2. Detach the radiator "in" transmission line, turn the car on and slowly put more new fluid in the dipstick tube (at about the same rate as what comes out the radiator line) while a mate monitors the colour of the fluid that comes out. Once it has a nice colour to it, stop pouring and put that radiator line back on, run it through all gears and check fluid level. Top up if necessary. (this is a more thorough way of getting the old fluid out, but can be difficult to sync the pouring and draining AND can be quite easy to over-fill)