OS X Yosemite: Turn off FileVault encryption
When you turn off FileVault, encryption is turned off and the contents of your Mac is decrypted. The decrypting could take a while, depending on how much information you have stored. However, you can still use your Mac to do other tasks while the information is being decrypted.
Choose Apple menu > System Preferences, click Security & Privacy, then click FileVault.
Click the lock icon to unlock it, then enter an administrator name and password.
Click Turn Off FileVault.
Click Turn Off Encryption.