Withdrawing STEEM POWER from your steemit.com account requires you to initiate a process so called Power Down that starts converting your STEEM POWER to STEEM tokens that you can transfer to other users or withdraw and sell. When you initiate a Power Down your current STEEM POWER will start to get converted in 104 equal parts and each of these parts will be available to you in the form of STEEM.
The first portion of STEEM will be available in your Wallet after exactly one week, then in another week you will get the second part of the 104 and so on with the process taking up 2 years or 104 weeks to complete. At any time you are able to Cancel Power Down from your Wallet the same way you initiated it, but the advice is to stop it once you have been paid one of the 104 installments.