You will have to pull the cover off by removing the staples.
Mark where you want to cut with a magic marker.
You can cut the foam using a large serrated knife, or even an electric knife. Roound the edges and smooth any irregular spots with sand paper.
When re-installing the seat cover, it will stretch easier if it is warm.
You will need a powerful staper to put the staples back in. A good hand held model will be strong engough. I think you want about 1/4" staples. Warm the seat base with a hair dryer or put it out in the sun to make it easier for the staples to penetrate.
Will it be worth it? Hard to say - on my old KDX250 it really helped improve the riding position.