Heater core

how do i replace my heater core on my vectra c 54 plate please???

You really need to get a shop manual if you intend to do this kind of work. There are a couple of Opel Vectra specific forums around the internet that can help you better. The one that popped up first in my google search, www.vectra-c.com/forum/index.php, seems to have a lot of car specific info just for your model car.