Hi kooroongah,
This will depend on the reason you are needing to make this change. If you are currently preparing a BAS and believe the total figures it is calculating are incorrect, you will need to review your reports to determine why. When preparing a BAS, the GST Centre will include all transactions from that time period with a GST code and any amendments from previously lodged BAS periods. We would suggest running some reports such as the GST Detail Report, Transaction Detail by Tax Code, Transactions without GST and/or GST Amendment report to determine where the discrepancy lies and handle it accordingly. If you have amendments from a previous BAS affecting the current period, you can reach out to your accountant for assistance in how to account for these.
If instead you need to make an adjustment for a BAS that has already been marked as lodged, you can either enter an adjusting journal with associated tax code or edit an existing transaction from within that time period. As long the transaction/journal is dated within that lodged period and lists the corresponding tax code (depending on which amount you are adjusting), the system will record it as an amendment to a period already lodged and it will show up as such when you prepare your next BAS.
Specifically how to make the adjustment will depend on the reasons you are needing to record this. Feel free to reply if you have further questions as we are here to help, we would also recommend you speak with your accountant or bookkeeper to determine the next step for your books.
-Kass
