To change the VAT code historically on a transaction it is a case of clicking into the transaction itself and amending the code and saving. In this case you would need to click into each transaction that should have been 0% and change the code and save the transaction.
You can going forward set default VAT codes on a chart of account if it is for a specific expense/income account
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However if you have an accountant who has access to QuickBooks as an accountant user they can use the reclassify tool to move transactions from one code to another in bulk.It is not a function available in the small business product yet.