Hello prickettjudith,
When you submit an open return, you should be able to amend the dates from the Prepare VAT return section in QuickBooks. You'll be able to change the accounting period, end date, and submission date before submitting them over to HMRC.
Please refer to this screenshot for your guide:
For additional insights into VAT returns, such as the filing, I recommend visiting the article I'm adding below:
Submit a VAT return in QuickBooks.
Feel free to get back to me anytime by posting any response concerning your VAT. I'll be here to help.