how to turn off firevault?

From the great Carolyn Samit...

Turn off FileVault

If you no longer want to encrypt your startup disk, you can turn off FileVault:

1. Choose Apple menu > System Preferences, then click Security & Privacy.
2. Click the FileVault tab.
3. Click the Lock button, then enter an administrator name and password.
4. Click Turn Off FileVault.
5. Restart your Mac.

After your Mac starts up, decryption of your startup disk occurs in the background as you use your Mac. This takes time, and it happens only while your Mac is awake and plugged in to AC power. You can check progress in the FileVault section of Security & Privacy preferences.

Use FileVault to encrypt the startup disk on your Mac - Apple Support

https://discussions.apple.com/thread/7613467

It must be Turned Off by the User Account used to Turn On FileVault