In shopware 6 you can adjust the tax rate on existing orders by following the below steps.
Step 1: Log in to the admin panel and go to the Orders > Overview.
Step 2: Click on the View option of the order.
Step 3: Now click on the Edit button to edit the order and adjust the tax rate.
Step 4: Go to the Items section and double-click the VAT section to edit the vat rate of the order.
Step 5: After adjusting the VAT rate, Click on the Tick button to save the VAT value.
Step 6: And click on the Save button to save the order.
Please contact us at manish@bay20.com or call us at +91-8800519180 for any support related to shopware. You can also visit the Shopware development page to check the services we offer.