Important - Read before proceeding: if a sales tax rate changes, always add a new rate instead of changing the original rate. If you do so, any old document edited would use the new rate.
To add a sales tax rate, click Settings then click Manage lists (units, taxes)..
Click Edit in default sales tax:
In the next window, you can also reorder the list and delete the rates you are not using :
Note : if you need to use two sales sales tax (eg: local and state taxes), you can define this option in Settings then Document settings.