I have promoted an employee, so their pay has changed in this current months salary period but cant seem to pay them 2 different rates in one month?

Hello there, jenbatchelor.

I can help discuss some insights on how to handle salaried employee with different rates in QuickBooks Online (QBO).

Using two different rates when paying an employee in QuickBooks Online is currently not an option. However, you can manually override amount from your employees paycheque.

On the other hand, since your employee changes rates, you can follow the steps below in case you need reference on how to your edit employee information:

1. Go to Employees from the left menu.
2. Select the name of the employee for whom you'd like to either edit a pay rate.
3. Click Edit employee.
4. Click the pencil icon from the Employment details section.
5. Enter the correct Pay rate.
6. Click Save.

You may also find the following articles handy for future reference: 

• Learning about QuickBooks Online Payroll for UK.
• Edit employees in QuickBooks Online Payroll (UK).

Should you need more help with this or any payroll questions,  feel free to get back to me and I'd be glad to assist you further.