In order to change, please do the following:
Navigate to the Debtors tab.
Find the Records section.
Click Analysis.
Select the analysis code to set as default from the list.
Click the Red Pin icon in the Record section called Default.
Note: when using the module menus view, the red pin button is located within the Sales Ledger - Analysis window itself. The analysis record now displays an extra tag showing that it is the default analysis code.