It is easy to change something, just tap it. If you want to change the VAT settings tap on the amount at the top of the screen. You can't change an invoice by changing a customer or item record, you need to change the information on the document.
You can change your VAT settings by going to "Settings", "Company settings" and "VAT". If you created an invoice or a quote without VAT and you need to turn VAT on, then delete the invoice or quote and do it again.