On the Web App navigate to Account ->Tax Rules.
Select Value Added Tax and enter the new tax percentage.
Next, click on the 'Update' button to save the new percentage.
All the Transactions that were captured on the Mobile App before the change will still reflect as having the original VAT percentage associated with them.
After the change, should new Invoices be opened on the Mobile App and new Deliveries be captured on the Web App these will all reflect the new VAT, without additional intervention.
Should there be existing or unconfirmed Orders, Deliveries, Issues or Reductions in the system they can still be committed with the old VAT percentage, by simply selecting the confirm arrow next to the delivery.
Unconfirmed Delivery after VAT change with 15% VAT still reflected: