I have a US passport travelling from Guam to London. My first transit is Seoul (Incheon Airport). Lay over for 10hrs before my next flight to Dubai. Is there any minimum days to stay in Seoul before my next flight. Can I leave the Incheon airport for sight seeing then come back for my next flight.
There is no such thing as a minimum time you are required to stay in South Korea (or, as far as I've ever heard, any country anywhere).
You also have a passport with which you can enter Korea without any sort of visa for a short visit, so you will have no problem there.
Since you have only 10 hours between flights, you should consider the possibility that you don't have enough time to clear immigration and security going both ways, and also do any significant tourism in the city. That said, you can stop by the Transit Tour desks in each terminal to see if they have a short tour which will fit in the time you have available.
Allow 2.5hours between city and airport both ways (take the rail/subway, 75mins + 75 mins customs) and you should have 5 hours in the city. It's more than enough to see Hongdae, Gangnam or Jongro (main city areas) but not all at once.
Pick one area, grab a meal there and explore! 10 hours is enough.
There are free transit tours from the airport too. Take them up!