How can i fit a new door seal gasket in my hotpoint WMA11P washing machine?

my current door seal gasket has developed some mould, so i want to replace it with a new one. i do not know how to remove the door seal retainer to get the old one out, and then i'm worried i wont be able to secure the retainer down again around the new one...! would welcome the wisdom of those that know how it all works! ta.