Hello biyv,
The Reclassify transactions tool is only available in the QuickBooks Online for Accountant version.
If you have an accountant that uses QuickBooks Online for Accountants, you can add them as an accountant user in your QuickBooks file and, they can reclassify your transactions. Otherwise, you'll have to edit your transactions one by one to change the GST..
You can check this article about how the reclassify tool and how to add an accountant:
If you have more questions with QuickBooks Online, please feel free to reach out to us at any time.
