Trying to replace o ring

Hi, I have a pentair 2000 filter that needs new o-rings. I can't figure out how to remove the pieces in the casing to replace them. Does anyone know how to do this? I found the instruction manual on line and it says to replace them every year, so it must be simple. I just don't know how. Thanks,

What's simple to someone who's handy is not so simple to someone who's not. My wife still can't figure out how to use the DVD controller!

I'm not familiar with that filter, but I'm moving it where someone who is can possibly answer.