To power down your Hive fully you must wait for 13 weeks. This is built into the code of the blockchain and there isn't any way to speed this process up.
It's important to note that having your Hive as Hive power is a great thing and powering down might not necessarily be the best option for your Hive stake. Having more Hive power increases your vote value, which in turn increases your curation rewards. It also gives you more weight in voting for proposals or witnesses which overall provides you with the ability to have more influence on how you would like to see Hive progress.