If you need to amend a rate within an already drafted invoice, you are able to do so by heading to Finances > Invoicing > Invoices > [Select the invoice]. You will be able to amend the necessary rate by clicking on it and updating the info. Click Save and you will get a confirmation message in the bottom right hand corner.
You can do the same for a Payslip by heading to Finances > Payroll > Payslips > [select a payslip].
You will not be able to change the rate after the invoice has been issued.
Below is a walk-through video of how to get change the rate within an invoice.
If at first the video is a blurry, ensure it has enough time to buffer fully.