You can edit the transactions that caused incorrect figures on your BAS, cazb. I'll guide you to get the transaction list.

You can open the GST Liability report and set the date range to the concerned tax period. Then, click the amount of the tax that needs to be changed to show the transactions that comprised it. Then, open the one that needs to be changed.

After the edits, you can lodge your BAS.

You can go back on this thread if you have additional questions about your BAS.