If you don't want a mess on your garage floor and don't feel you are technical enough to handle the Bay 13 method, using a fluid pump to get a portion of the transmission fluid helps. You may want to do a few successive attempts to draw out the transmission fluid as any one try will only pull out about 1/3 of the trans oil. So doing will replace the transmission fluid, but not the debris on the bottom, if any. But for ages, prior to all these fluid flushing machines, auto mechanics routinely only dropped the transmission pan, replaced the filter, and replenished only about 1/3 of the fluid. It was called "transmission service".

Now the 850 doesn't have a filter and no band or valve adjustment is needed. It seems that replacing the trans oil more frequently is even more important.

But a transmission fluid flush seems to be the better alternative although it is much more expensive than the DIY project.