Customer service does have instructions on how to remove the fabric from the back of the seat, but assuming you have a Crypton fabric (I see pink in your avatar, so if that's the seat involved it's crypton,) then I wouldn't jump to removing the fabric right away.
The fabric itself is impermeable - nothing can get through the fabric to the seat cushion underneath unless it somehow went through a harness slot hole or something like that, so you don't have to worry about there being anything "underneath" the dirty area to clean.
Crypton is the only fabric that is able to be completely disinfected because every fiber is coated and it has an impermeable backing on it, so assuming that it's soiled in an area that is manageable so far as reaching everything, I would first try to clean it without removing the fabric.
I've tested the crypton fabric on my seats and water really beads and runs off it instantly though, so I'd suggest putting some paper towel or towels around the area to catch any excess water. And I would probably actually start with a baby wipe first. A baby wipe will probably clean most of it off and then you can use the directions in the manual.
Sorry you're having to deal with this, explosions suck. Customer service isn't going to be open until Tuesday due to a long weekend, but I might be able to get some more specific instructions for you sooner in terms of removing the fabric if this doesn't work - just send me an email through the forum.