In order to change your ABN in the employee tab you will need to go through the following steps:
1. Login into Quickbooks
2. Click on the "Employees" on the left hand menu side.
3. Click on "Payroll settings" followed by "Details"
4. You can change the ABN under the ABN settings
Note: Once the ABN is changed in the details field, that will change the ABN in All payslips going forward, and to the ATO when submitting Single touch payroll (STP).