Hello Ryan:
I agree with Steve, that the easiest method is to start over by issuing a CR Note and redoing the Invoice.

If this is an OE application, another thing that you could do is to set up a Misc. Charge called Tax (or GST/HST) Adjustment and point to the appropriate GL account. (We had to do this with a client that had to adjust quite a number of Invoices that had the wrong tax applied, but they wanted to retain the original Inv #'s.) If you go with this method, make sure that you set this Misc Charge as NON-TAXABLE!