I may be wrong, but wouldn't it be better to de-activate your old rate & create (activate) a new item for your new rate. If not, all your pre-new rate invoices will not retain the old rate description.
Thank you for adding a reply and sharing you thoughts, @Anonymous.
Yes, we can change the sales tax rate by going to the item list and locate the sales tax item we wanted to edit. However, we also have the option to deactivate the old sales tax item with the old rate and create a new one.
Both deactivating and editing the sales tax rate won't affect the previous transactions/invoices created using the old rate. For the detailed process on how to edit the sales tax rate, refer to the steps given above. On the other hand, to inactivate the old sales tax item and rate:
Go to Lists.
Select Item Lists.
Locate the Sales Tax Item.
Check the box beside Item is inactive to inactivate the item.