The configuration file is called applicationhost.config.
It's stored here:
My Documents > IIS Express > config
usually, but not always, one of these paths will work
%userprofile%documentsiisexpressconfigapplicationhost.config
%userprofile%my documentsiisexpressconfigapplicationhost.config
Update for VS2019
If you're using Visual Studio 2019+ check this path:
$(solutionDir).vs{projectName}configapplicationhost.config
Update for VS2015 (credit: @Talon)
If you're using Visual Studio 2015-2017 check this path:
$(solutionDir).vsconfigapplicationhost.config
In Visual Studio 2015+ you can also configure which applicationhost.config file is used by altering the <UseGlobalApplicationHostFile>true|false</UseGlobalApplicationHostFile>
setting in the project file (eg: MyProject.csproj). (source: MSDN forum)
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