web.config updating programatically

Hi
I have a web config setting which I retrieve :
ConfigurationManager.AppSettings("cboMaxEmployeeHoursPerWeek")
this brings back the value i can read by opening web.config
If I use the website asp net config manager I can change this
 If I use the following code:
ConfigurationManager.AppSettings("cboMaxEmployeeHoursPerWeek") = 20
or
ConfigurationManager.AppSettings.set("cboMaxEmployeeHoursPerWeek", "20")
the web.config file does not update
 why please?
 
VS2008 Vista
Microsoft Visual Studio 2008
Version 9.0.21022.8 RTM
Microsoft .NET Framework
Ve ...