The transmission will have two lines running to the radiator, where there is a heat exchanger (little radiator) built in to cool the ATF.
The idea is to remove the return line, slip some rubber hose over it and put the end of the hose in a five gallon bucket.
You run the engine at idle and add ATF at a rate to keep up with what's getting pumped out.
You continue this until the ATF comes out clean.
You'll need probably double the ATF capacity of the transmission on hand, maybe a little more.
Before doing this, remove the pan, if there is one, clean it and replace the filter.
Bet the pan will be loaded with clutch material and the magnet will be hidden under a large blob of metal paste.
This is the best you can do to maintain your automatic.
Fresh fluid can transform an aging automatic.
If it dies, it was going to anyway.