Calling UserControl method from MainWindow

Question

I'm new to WPF and learning from day to day. Figured out a few things.
But one (simple) thing I don't know how to manage it the right way.

Classic scenario:
MainWindow which has a few UserControls added. Nested.
1, Running the app I choose File->Open. After selecting a Filename I press enter. From the dialog I got the filename. We're in the On_Click auf the MainWindow.
2, Now I'm trying to "send" this filename to a usercontrol which is nested somewhere in the "tree" to do some work.

I read about RoutedEvents, RoutedCommands, delegates, events and so on.
After days I'm still confused what's the right way for me to go.

Hope to get a little advice from you.

Thank you,
Bernhard