I think it depends on how big, how long and that sort of consideration..but in general it should be seen by a vet.
In the mean time, I would clean the sore with some mild soap and water, perhaps using a gauze cotton ball or pad, something I can throw in the trash. Then I'd use some hydrogen peroxide to further clean, again using cotton balls, perhaps trying to flush/flood the spot by dunking it into a cap of hydo peroxide. I do this every few hours and if the condition didn't improve I'd take the dog to the vet. There are over the counter animal products for minor scratches that may provide some additional help.
I say all the above assuming the condition is minor, a fact I can not determine by reading a brief post.
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