Have the transmission drained of all that can be removed, then filled to the correct level. This will change about 50% of the fluid, which is the best that can be done with modern transmissions that have no pan and no means to drain the torque converter (TC). What some have done, including me, is to drive it until the next engine oil change, then drain and fill the trans again. This will (mathematically) yield 75% new fluid in the trans. Doing it again would be 87.5%, etc.
By all means, do NOT flush the transmission! What @1966tbird
brings up seems to be a good idea, that of flushing as much fluid as possible by means of pumping it out while replacing it with new. However, GM advises against any form of flushing, whether by using an external machine that has a pump and the new fluid, or by starting the engine and letting the transmission pump do the work while someone adds new fluid. GM says that either method can lead to damage and they will not warrant a transmission that has been flushed by either method.
That leaves you with the safest course of action to be a drain-and-fill. Sadly, the TC does not get drained, so to get more of the old fluid replaced, you'd have to drain-and-fill multiple times to get a higher average ratio of new fluid to old fluid.
Starting with the 2008 models with the 6-speed transmissions, there is no user serviceable filter, either! The screen is permanently trapped between the 2 halves of the case. To replace it requires a full tear-down of the transmission.
Imagine if changing the oil on the engine no longer included a new filter!
For over 10 years, owners have lamented this stupid design because it takes away the ability of those who are conscientious about maintenance and forces them to deal with what amounts to an appliance that is tossed when it's old rather than having a machine that can be maintained and serviced.