i. Go to Set-Up → Sales Invoices
ii. Select the sales invoice for which tax has to be changed and click on ‘Edit Sales Invoice’ button from tool strip
iii. The same sales invoice will be open in billing screen. Click on ‘Change Sales Tax’ button from tool strip (If you cannot find ‘Change Sales tax’ button in tool strip click on ‘Manage tool strip button’ and shift the ‘Change Sales tax’ button to tool strip. ‘Manage tool strip button’ is the last button in the billing screen’s tool strip)
iv. Select the tax and click on ‘Apply To Selected Items’ or ‘Apply To Complete Sales Invoice’ based on your requirement
v. Complete the invoice by entering payment received and saving it.