To change the Tax code, this has to be changed under the Details screen for the current job. If the Job has a Finalized Invoice and or a Completed Proposal, then those will need to be undone:
If there is a Finalized Invoice, you will need to press the UNFINALIZE button.
Then you would need to navigate to Proposals Tab and first press the Uncomplete button and then the Decline button.
Now you can navigate to the Details tab the Tax Code Dropdown will be editable.
Also, you can set a permanent Tax Code for a Property.