How do i change tax rate from 23% to 21% ?

Welcome and thanks for visiting the Community, @apollogalway.

Changing sales tax rates in QuickBooks Online (QBO) is pretty quick and easy. This can be done by going to the Taxes menu.

Here's how:

1. In QBO, go to the Taxes menu at the left pane and select the correct tax agency (if multiple).
2. Click the Edit drop-down at the top right, then choose Edit rates. 
3. Pick the rate you want to change, then Edit.
4. Change the rate in the Edit tax rate window and set the Effective Date, if required. This will be the date from which the rate change will take effect.
5. Hit Save to complete.

To know more about what will happen when you change a tax rate in QBO, check out this article: Edit sales tax rate.

There are also certain situations affected by these adjustments. For more information, please refer to this article: Manage a tax rate change.

Reach out to me in the comment section below if you have any other questions or concerns. I'll be more than happy to help. Have a wonderful day!