VAT credit is to be carried forward to the next month to offset the VAT payable in the current month. How do i adjust this in the VAT return which has already been filed?

Hey there, @pankhaniaco.

I can see that this is your first post here in the QuickBooks Community. Therefore, let me take this time to welcome you to this public forum.

Since you have already filed your tax return, the program will mark it as Mark as filed. 

However, you can still unfile a tax return to make necessary adjustments. All you'll need to contact QuickBooks support and provide the following information:

1. QuickBooks Online Company ID.
2. Which tax filing period you wish to unfile.

To connect with our QuickBooks Phone Specialist, follow these steps:

Option 1

1. Click the Help icon.
2. Hit the Contact Us button at the bottom. 
3. Enter your concern in the description, then click Let's talk or Continue.
4. Select Get a callback.

Option 2

1. Click the Help menu in the upper-right hand corner.
2. Type in "Talk to a human", then press Enter.
3. Look for I still need a human and click on it.
4. Click Contact Us.
5. Select between Send a message, Schedule an appointment, or Get a callback.

The unfiling process is carried out by the back-end team. During this period, all users must remain signed out. Just click the Gear icon and select Sign out. Once the tax return has been successfully unfiled, our support team will contact you.

For more details, you can check out this article: How to unfile VAT/GST.

Let me know how else I can help you with QuickBooks by adding a comment below. I'm more than happy to lend a helping hand. Keep safe!