two ways:
1- send to an exchange (like bittrex) and exchange for bitcoin. Then you can change bitcoin for whatever coin you like, including steem.
2- If you only want steem, then you can just click the arrow next to the amount of SBD in your wallet. Then in the drop down that appears click on "market". This will take you to the steem internal market, where you can sell SBD for steem at the real price (coinmarketcap). If you don't know how to use the internal market, there are many posts and videos explaining this, google it!
Yes but when I click on steem dollar then « transfer » I can only send it to an other steem account
Yes send it to bittrex. You should first sign up to bittrex, then click + on your steem dollar wallet. It will provide you with the memo and the account name (which is @bittrex).
Ok thanks !