I was wondering if anyone can give me simple instructions of how to change an Acquifier water filter cartridge? I believe I have to turn the water off first, under the sink, and need to locate a quarter turn valve. I was wondering if anyone can help me locate this valve or a point under the sink where I switch off water so I can replace the cartridge. I assume I also need to switch off the mains water point. H ere is an image of under the sink - the think blue piping leads to the cartridge that needs to be replaced. Thank you!