I'm not that familiar with SC but assuming there isn't a native way, you could sign up with an exchange that has SC (the major ones like Binance, Kraken & Bittrex do). Then convert it to fiat there and withdraw. Most exchanges have API's so this could be automated.
There are also various decentralised exchanges that handle pegged-fiat crypto currencies for price-exposure.
But after a while you may regret doing that, as much of this tech is in very early stages, and as such the value of the coins are low versus their longterm potential. If you believed in Sia/SC longterm, it might be better to hold the SC and convert to dollars later.
For an example of why this may be the case, look up bitcoin pizza guy.
(And obviously none of this is intended as investment advice, always do your own research, etc.)
