Hello, office19.
You might have been using the automated sales tax that's why you won’t be able to edit the 2017 taxes. You’ll have to contact our phone support team to change it back to the old tax center. Once changed, undo reconciliation for 2018 and 2017 then manually change the tax rates for each affected transactions in 2017.
Let me know on the comment section if you have other questions or you need assistance with QuickBooks Online.
