You can convert your Steem to fiat currency by transferring your Steem to an exchange. Selling it to Bitcoin and using an exhange like Circle.com or Coinbase.com to sell to fiat.
If you've already sold your Steem to Steem dollars, you can also send your Steem dollars to an exchange like Poloniex, sell to Bitcoin, and sell on an exchange that buys Bitcoin for fiat like Circle.com or Coinbase.com for fiat.