If you've uploaded the list to a new static list it'll be easy. Just run a smart campaign with the filter "member of list = the one you created" in which you uploaded the list; and use "Change Data Value" in the flow to change their lead source.
Hope this helps!
Sorry to take so long on this. I have it all set up, but have not run it yet. I want to make sure that nothing is going to be sent to that list, that running the campaign only will change the lead source, Correct?
That's correct. As long as you don't have any specific flow steps that would include such activity (such as "send email"), you're OK. This would probably be rare in this case, but you also want to make sure there are no other trigger campaigns that do something as a result of a specific "change data value" flow step. But worth mentioning, regardless.