Hi Dee,
AXatak already replied where to setup the database log. What you still need to figure out is the table that needs to be logged as this table need to be setup in the database log.
A first step for finding out which table needs to be logged is related to your process. Are you recording AR free text invoices, AR sales order invoices ... for which you want to have the changes in the payment terms tracked? If so, then you need to find out and specify this table in the database log. If you do not know how to find out the underlying table, ask a colleague from development. They will certainly be able to help you.
Yet be careful with such a setup especially in regards to system performance. In an ideal AX world you should try avoiding logging transactional data such as sales order transactions, vendor invoice transactions and alike. So, please test in detail whether and how the database log negatively influences your system performance.
All the best,
Ludwig