You can change VAT easily. If it is the same all the time, change it from the invoice settings (Settings - Invoice Settings - Invoice row).

From there you can choose or enter your preferred VAT rate:

You can also choose whether or not VAT is shown on the invoice row, and whether the prices are with VAT or without VAT:

If you wish, you can also do all this when preparing the invoice and even choose a different VAT for each row of the invoice. To do this, click on the unit price field and select your desired settings: