Hi Accounts95, 

Thank you for reaching out on this. 

To adjust a BAS, we would need to find out if any transaction(s) need to be adjusted as BAS figures are pull from transactions with GST codes. If you are able to identify the transaction(s), all you need to do is to make the change on the transaction(s) and QBO would capture, and include the change as amendment in the upcoming BAS. 

Hope this helps. 

Please let us know if you have any follow up questions.