modification of the config file using msbuild. Please help me its very urgent

Hi All,

           I have a C# solution file and i am using Msbuild as a build runner. I have created 2 system properties in the build configuration and i want to modify the app.config file by these properties values.

Could you please let me know, how to modify/update the config file using msbuild script.

Thanks & Regards
Kiranbabu.A