Update Property Value on a Doc to reflect value on Object

I have an object called "Plan". On the Plan's metadata card I have a property called "Document Owner".

On my Document object I have the Document Owner property on there that populates per the Plan selection.

Sometimes we may have to change the Document Owner on the Plan object. How do I make this change reflect on my existing documents? (in my image below I would expect to see Josh name rather than Ronda).