Select the highest buying order price by clicking on it. Enter the quantity of DEC you want to sell in "Sell DEC" "Quantity". Click on "Sell DEC". Confirm by clicking "SELL" when prompted.
Go back to the top of the screen and click on "WITHDRAW"
Enter the amount of HIVE you want to withdraw in the "WITHDRAW AMOUNT" field (likely to be the value of your "Current Balance"). Click on "WITHDRAW HIVE".
After a minute or two, the HIVE should be available on your account.
How to: convert your Splinterlands' DEC to HIVE | Ecency