Etz will not allow you to amend just the tax element of a timesheet.
The procedure to make an adjustment which just affects the tax of an invoice is by posting one costing line with the tax element and then a credit costing line without tax.
The following example adjusts the tax (VAT) by 2p
From edit Invoice amend the timesheet by adding the two adjusting lines:
you will be required to Override the warning message and then recreate the Invoice document: