Thanks for posting to the Community, @Buck Tsai.

You won't be able to undo the tax filing. However, you can email to our Care Support Team to unfile the filed tax return.

They have tools to pull up your account and help you unfile filed VAT. Please make sure to provide your QuickBooks Online Company ID and the tax filing period that you wanted to undo the tax filing.

You can see your Company ID under Billing and Subscription in Account and Settings by utilizing the Gear icon.

Here's how to email our support:

1. Go to the Help icon on the upper right.
2. Click Contact Us.
3. Enter "Unfile tax return" and then hit Let's talk.
4. Choose to Start an email.
   

Also, I'll add this article as your reference in accordance with the South African new rate: VAT Feature Update FAQ.
 

Please let me know if you need further assistance. The Community is always here to help. Take care!