This article describes how to select a different tax rate on your invoice, and also how to create a new tax rate while you are creating an invoice.
This has screen shots for both tablets and phones.
On a tablet, tap on the blue chevron next to the existing tax rate.
On a phone, tap on the Pick button on the Tax heading line.
This brings up a list of your existing tax rates. Simply tap on the tax rate you want.
Or if you need to create a new tax rate, tap on the Plus in the upper right corner.
Enter your new tax information and press Save.
The new tax rate appears in your tax list. Tap on it to select it.
The new tax rate is on your invoice.