Taxes can be applied to orders once they have been created within the system. There is an article here which outlines how to create taxes. You also have the ability to select a default tax rate when creating your taxes, so this rate would apply to orders automatically. However, if a default isn't selected or if you need to change a rate for any orders you can definitely do so.

There is an option for tax within all estimates or jobs and it can be found above the total amount of your cart. There is a pencil icon beside the tax which allows you to click to either apply or change tax.