To remove a tax from an a sales order invoice, you just have to do the below steps.
Go to the Sales screen > Search for the SO number > Open the Order
...
Click the menu icon > Click Sales Order Settings
A pop up will appear. Make sure you make the tax 0% here
Hit Save once done
Once that is done you will notice the tax has been removed.
Click GENERATE SO at the bottom of the screen to save your changes.
In the SO screen click the Invoice Tab, and open the Invoice number.
...
In the right upper corner, click the MENU icon, you can see the Sales Order Settings, click the settings.
...
In the screen Update the Sales Tax % to be 0.00
Save the changes by clicking the Save button.
Close the Invoice screen.
Re-open the invoice number (top right), and it should load and populate the changes. Tax should be 0.00
...
.
Click RENGERATE INVOICE at the bottom here so it updates the PDF so you can send it out to the customers.
...