Click the Settings icon in the upper-right corner
Under Company Setup, click Tax Rates
Find the tax rate to delete and click the garbage can to the right of it.
If you receive a message, "Cannot delete record because of dependent service_locations" it is because that tax rate is already in use and cannot be deleted.
If we allowed the deletion of used tax rates it would make all associated records invalid. As a work around, we can update everything to a single tax rate.