Thanks for posting here in the Community, @ribbyone. 

Yes, you can adjust your current VAT account to correct errors on past returns. The following conditions should be met:  

• The net value of the errors is £10,000 or less (reporting threshold). 
• The error amount is up to 1% of your box 6 figure (up to a maximum of £50,000).
• The accounting period ends not more than 4 years ago. 

You must report any errors that do not meet the above scenarios to HMRC. For more information, check out this link: Correct Errors. 

Please take note that any manual adjustments will impact your VAT control or VAT suspense account. With this, I'd suggest consulting your accountant before performing the correction. He/she will properly guide you on how to adjust the boxes on your next return. 

Once you're ready, here are the steps on how to adjust VAT box 1, 2 and 4: 

1. Go to Taxes from the left menu. 
2. Select Submit VAT for the period you are submitting for on the VAT page. 
3. Click Adjust in the VAT line that you want to change.
4. Select an Adjustment Date and VAT rate. 
5. Choose an account from the Adjustment Account drop-down list. 
6. Enter the adjustment amount in the Amount section. 
7. To describe the adjustment, enter a memo. 
8. Click Save.

To proceed with the other steps, see the To adjust VAT box 6, 7, 8 and 9 section through this article: Adjust Your VAT Liability.

Once done, the program automatically adds a journal entry to show the adjustment. When you file your next return, this will include the adjustment.

You can run the VAT Detail report to ensure your VAT return is correct. 

I'm here anytime you have other concerns. Have a great day.