The FAQ shows you how to at/update the GST/VAT rate in Spectra PF.

Please see below instruction;

Users can set up/update the GST/VAT tax rate at a division level in the VAT Rate field (screenshot below). Once updated, please remember to save your change.

The new rate will apply for all new relevant transactions created after the set-up/change to the GST/VAT percentage. 

Please note, all relevant transactions created prior to the change will carry the old percentage rate of GST/VAT. If you require any of these transactions to have the new rate applied, you will be required to generate a reversal. 

By using the right-click option from the grid screen, you can generate a credit and the old rate of GST/VAT will automatically be applied to it.