There are 3 types of stock costing method in SQL Account.
FIFO
Fixed Costing
Weighted Average
Go to Stock.
Select Maintain Stock Group.
Assign your preferred costing method for related Stock Group.
#TIPS: If you CHANGE your costing method (previously with existing transaction), please remember to run Analyse Data Integrity before you check the stock costing report again. SQL Account will auto update from first date until current date transaction with the new costing method.