Open System Preferences and click Security & Privacy. Under the FileVault tab, click Turn off FileVault.

1) Open System Preferences.

2) Click Security & Privacy.

3) In the bottom left-hand corner, click the lock icon.

4) Authenticate using Touch ID.

Alternatively, you may type a username and password with administrative privileges on your Mac. Then, click Unlock.

5) Under the FileVault tab, click Turn Off FileVault.

6) Click Turn Off Encryption.

7) Wait for your Mac to decrypt the disk.

NOTE: The amount of time for your disk to decrypt will vary with the amount of data your disk contains and its size.

Outcome

Your Mac has been decrypted.