I'm not able to change back the passcode from 6 digits to 4 digits.

You'll find several settings and options:

• Turn Passcode Off: Tap this option to turn off your passcode.
• Change your passcode: Enter a new six-digit passcode. Or tap Passcode Options to switch to a four-digit numeric code, a custom numeric code, or a custom alphanumeric code.
• Require Passcode: By default with this setting, as soon as you lock your screen, you need to enter your passcode to unlock. If you don't want an immediate passcode requirement, change this setting (For your security, if you use Touch ID or Apple Pay, you can't change the immediate passcode requirement).
• Allow Access When Locked: Use this option to allow access to some features when your device is locked, including Today View, Notifications View, Siri, Reply with Message, Home Control, and Wallet.
• Erase Data: Choose whether to erase your device automatically after ten failed passcode attempts.

When you try to turn off passcode you will be prompted to enter your current passcode before it will switch off. Are you seeing that prompt ?

If you have a corporate Exchange account on your phone, the administrators can enforce certain policies on your device including the requirement for a passcode, minimum passcode length, etc. If that's the case, you will not be able to remove the passcode or change the passcode length until you remove the Exchange account from your phone. If you re-add the account, the rules will once again be enforced.