How do I turn off encryption

To turn off FileVault encryption you can follow the steps here:

Turn off FileVault encryption on Mac - Apple Support

Be aware that depending on the capacity of the drive and the amount of data that needs decrypting, the process may take some time, days even. But it all happens in the background and you can continue to use your Mac while it decrypts the drive.

System Preferences >> Security & Privacy >> FileVault and disable it.

One will have to wait for it to finish decrypting.