You can modify the end date of your return when preparing it before submission. Let me show you how.
Learn more about what you need to prepare before you submit your return with this article: Submit a VAT return in QuickBooks Online.
Additionally, I've also included this article for the steps in recording a payment or refund for your return: Vat Payment or Refund.
If there's anything else that I can help you with, please let me know in the comments below. Stay safe!