Not that any cat would do such a thing, but if you get cat poop on your upholstery, here's a reliable way to remove it:

1 - The obvious step, pick up and remove the big pieces as you are able to. You may want to consider wearing some cleaning gloves for this.

2 - Blot, but DO NOT RUB, the stain with some damp paper towels. Just push the towels onto the stain to get the stain slightly damp.

3 - Sponge on an enzyme-based pet odor eliminator. I use Outright Pet Odor Eliminator. This has a bacteria which actually breaks down the material left by your cat.

4 - Cover the area with a plastic bag or sheet to keep it damp.

5 - After a couple of hours, your stain should be gone. Blot with some more damp paper towel. If there is still some residual stain at this point, try use a carpet stain cleaner to remove the last little bits.