If you are using QuickBooks, you will need to add or modify the tax items in your QuickBooks company file first. Then, open the Core5 preferences and re-import the Taxes. This will not delete any existing taxes, but will update existing taxes and add any new taxes into Core5.
If you are not using QuickBooks, go to the Taxes tab in the Core5 Preferences. You will see a '+' button to the left of the labels. Press the '+' button and enter in the tax name, tax description, and rate.