Hi Louisa,
Thanks for posting here in the Community.
We have to unfile the VAT return in order to change the dates. Our support team can help you do this process.
Here's how to reach them:
- Click Help
- Select the Contact us link.
- Choose VAT as the topic.
- Click Send Email.
Once done, you'll have the option to edit the dates then mark your VAT return as filed.
Let me know if you need anything else. I'll be right here to help you.