It should have come with an instruction booklet that shows how to remove the cover and how to properly install it in a car, etc.
I googled the carseat and came up with this website
http://carseatblog.com/?p=1066
I copied this from that website:
Owners Manual: The owner’s manual is large, clear and has good diagrams and pictures. It is conveniently attached to the seat with a cord but is easily removed without having to cut the cord.
Cover Removal & Fabric Care: Removing the cover takes effort due to two plugs on the outside that hold the cover in place. These plugs are, in my opinion, unreasonably difficult to remove. The cover itself is hand wash and line dry only. If you need to get the cover off and you’re having trouble removing the plugs – use a butter knife to pry them out just a little and then use pliers to finish the job.