Answer: Preen is a pre-emergent herbicide so it will stop weed seeds from germinating but it won't kill established. Instead, you'll need to dig out the weeds and their roots and then apply Preen.
Since the weeds are between pavers in your patio you can use a broad-spectrum weed killer (Round Up is an example) and after the weeds have died down you can pull the debris up and then sprinkle Preen in the areas. That should stop future weed growth.
Best wishes with your landscape!
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