Can't Decrypt Harddisk on HP Drive Encryption

[ilhabibi]

Hi,

I want to decrypt my harddisk. For existing my C drive is enctyped by HP Drive Encryption, but I can't decrypt it. I read in help notes (I click the question mark on HP Drive Encryption window) I found this is how to decrypt the disk:
1. Un-thick the drive

2. Click apply

My problem is, there is no apply button so I can't do anyting about that.  Where is the button?if it doesn't exist, how to decrypt my disk?

I attach the screenshot 

Thanks in advance.

Ilhabibi

[ilhabibi]

Update : 
After I created kind of "spare key", somothing like that, finaly I can decrypt my disk through the guideline way. It means I just need uncheck the drive and click apply.

Thanks.