Hello @AngusSL,
You can modify the end date of your return when preparing it before submission. Let me show you how.
- Go to Taxes.
- Select VAT.
- Under Open returns, select the return you wanted to submit.
- Below the ACTION column, select Prepare return.
- Enter your desired end date.
- Click Submit.
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!