Moving Page Under a Parent to Enable It Appearing as a Subnavigation

Greetings!

I'm modifying the "Spirit" theme and am slowly catching on to how it all works. There are a few things where I'm still scratching my head though. The big one right now is how to move a page that I've currently got as a top level navigation as a 'child page' under another page. In other words, I want to create a sub-navigation.

I tried going to the WordPress dashboard and marking that page as belonging under the "parent" of another page, but it's not 'taking.'

Does this theme not accommodate sub-navigation? Or is there another way to do this I'm not aware of?

Many many thanks!

Anne