Open PowerShell with 'Run as administrator'. Then type the following commands.

manage-bde -status

BitLocker Drive Encryption: Configuration Tool version 10.0.16299
Copyright (C) 2013 Microsoft Corporation. All rights reserved.

Disk volumes that can be protected with
BitLocker Drive Encryption:
Volume C: []
[OS Volume]

    Size:                 476.34 GB
    BitLocker Version:    2.0
    Conversion Status:    Used Space Only Encrypted
    Percentage Encrypted: 100.0%
    Encryption Method:    XTS-AES 128
    Protection Status:    Protection Off
    Lock Status:          Unlocked
    Identification Field: Unknown
    Key Protectors:       None Found

Disable-BitLocker -MountPoint "C:"

   ComputerName: MyComputer

VolumeType      Mount CapacityGB VolumeStatus           Encryption KeyProtector              AutoUnlock Protection
                Point                                   Percentage                           Enabled    Status
----------      ----- ---------- ------------           ---------- ------------              ---------- ----------
OperatingSystem C:        476.34 DecryptionInProgress   8          {}                                   Off

manage-bde -off C:

BitLocker Drive Encryption: Configuration Tool version 10.0.16299
Copyright (C) 2013 Microsoft Corporation. All rights reserved.

Decryption is now in progress.

Wait for the decryption to complete. Check this by running the status command (manage-bde -status)