Incorrect invoice processing

Good Day,

in March/Apr19 our external bookkeeper incorrectly captured our sales invoices incl vat, where it should be without vat as its now showing in your VAT report, how would i go about removing the vat amount after it was allocated to the sales. should i deallocate the invoice from the sales and the reallocate it with the new invoices? or is there a much easier way were i could just remove the vat amount.