IIS Express Configuration File

The configuration file is called applicationhost.config It’s stored here:

My Documents > IIS Express > config

usually (not always) one of these paths will work

%userprofile%\documents\iisexpress\config\applicationhost.config
%userprofile%\my documents\iisexpress\config\applicationhost.config

Update for VS2015

If you’re using visual studio 2015 look here:
$(solutionDir)\.vs\config\applicationhost.config