Short answer - you don't :) You power up! You convert it to STEEM POWER from your wallet. Long answer ... well, you send it to a trading platform such as Poloniex or Bittrex (Poloniex disabled STEEM transactions for now) and sell your STEEM for Bitcoin, and then you move your Bitcoin to a wallet (Jaxx for example, on your phone) and then you go to your nearest Bitcoin ATM and change for your local currency. I suggest you google for more info! Good luck