There aren't any provisions to update or modify your configuration settings via Powershell or WMI out of the box though you could build your own implementation to do this.
The primary way people update this during runtime is to modify the configuration file itself. For ASP.NET this will cause the usual application domain drain stop (new requests are redirected to a new app domain whilst the existing appdomain waits for executing
requests to complete before being torn down). In the case of any other app where this behavior doesn't happen (i.e. Windows Service or Windows Forms application) then you can call Trace.Refresh to force the configration to be re-read. This updates the
configuration settings within the current app-domain.
Again you could create a component that allows the System.Diagnostics pieces to be manipulated at runtime (via Powershell, WMI or however you like) but you'll have to do this yourself - most people are happy to just change the config file when necessary.