If you shut off A and B and unscrew the blue basket very slowly, you will get a little squirting of water until the residual pressure bleeds off and that should be all. A bucket and a couple of towels will do the job fine.
You might want to consider having some of that plumbing reworked. That hose and clamps and the plastic fittings there are not proper materials for a domestic potable water system. What you have there is something performed by a DIY who apparently didn't know any better.
If the cartridge has really been in there that long and hasn't plugged solid, it really isn't needed and you should consider getting rid of it altogether. A cartridge like that is a perfect place for bacteria to grow. If you insist on keeping it, the basket should be chlorinated and filter replaced at intervals of 3 months or less.