It is possible to to do this with a Data Value Changed filter, but only if you know which field(s) you want to look at. There isn't currently a way to say "Show me any lead who had any change in the past X days".
It sounds like what you want to do is find leads in another system that had an Update, and then sync those back to Marketo. Is that right?
You would have to use logic on the external database to do this, although you might be able to run a batch with a webhook that could then initiate a sync, or call another flow to run a sync on the API.