how do i change cartridge ?

our shower classic aqualisia 200 thermo would not turn off - in the end need to turn of tap on feed behind shower wall. apparantly we need a new grey cartridge but when i went to remove the old one the water just poured through thermo and head, should i turn of the main supply to the house. this is my first attempt at diy plumbing and any advice would be very much appreciated thankyou.