Product review (4 stars): Removing the upper body from the lower part of agitator
If the upper and lower parts don't separate with a firm tug then look into the top of the agitator.  The upper part of the agitator in some models have 4 white tabs that are part of the lower agitator that have been melted down slightly to firmly hold the 285809 part in place.  (Done at the factory)
As some suggested you can use a dermal tool but I found it easier to insert a small bladed screw driver through the holes in the upper agitator body that are directly in line with these melted tabs.  My agitator had 4 equally space square holes.  I used a file to make the blade a little sharper but with a little prying I was able to lift the edges of the 4 white melted tabs.  Then the parts separated easily with a tug.