How to invoice for VAT only

I’m sure this is possible but how?
One of my VAT registered clients has sent out invoice with multiple lines of items but just realised that some of the lines did not show VAT at 20% when they should have. Invoice has been paid.
How do they now raise a new invoice showing just the VAT that was missed on the original?

I found these posts which are maybe helpful

Thanks @rhc
Just the answer we needed.

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