Newbie question: string replace

[usgog at yahoo.com]

I have a config file with the following contents:
service A = {
  params {
    dir = "c:\test",
    username = "test",
    password = "test"
  }
}

I want to find username and replace the value with another value. I
don't know what the username value in advance though. How to do it in
python?