VAT oddity in invoice

[greyhound]

I have recently changed the VAT in business settings to 20%.

The client sent me a copy of a client invoice and it is still displaying the 17.5% VAT rate.

I cannot for the life of me work out how to
(a) Find a template of any kind to see which variable the invoice is using
(b) Work out how the central VAT rate is not getting to the invoice

Any help much appreciated.
Thanks.