If you wish to change the tax rates that are on your settlements, then you can do this at any time by going to Accounts & Taxes > Account & Taxes Wizard. (Please make any changes to tax rates via the Wizard rather than the Account & Tax Mappings page.)
Any changes to your accounts and tax rates will apply automatically to any settlement on your dashboard that has not yet been sent to Xero/QuickBooks.
For settlements that have already been sent to Xero/QuickBooks, you'll need to refresh these settlements before they pick up the new rates. This 'refresh' option is on the action menu for each settlement, or the bulk refresh feature can be used to refresh multiple settlements on the dashboard at once.
Once refreshed you can re-send the invoice to Xero/QuickBooks so that the correct tax rates are applied.