To set your default tax area, (or if you have a change in your default tax area) you'll want to go to U-Utilities, J-System Setup, J-Sales Tax, A-Sales Tax Base %

From this screen-

Enter sales tax base % - this is the default tax percentage for customers who are not Tax Exempt
Enter default area - Tax Area is a five character field, used for a code to represent the Tax Collected, region or exemption reason
Tax Other Area - if using Shop Supplies setup on Work orders to automatically bill, is that charge taxable Y/N
Tax other area on EXP - in specific areas if a customer is Exempt on Parts but not exempt on Labor - are they exempt on Shop Supplies Y/N

You will get a prompt asking to update all customers with that default tax area from the old percentage, to the new percentage. Then you will be asked to update open work areas with that tax area as well. 

When you are done updating/setting up the default tax area, you will need to go into customer files, go into ctrl/t area and change the tax amount in this set up as well.