how to recalculate from child form to parent form

to any technical axapta im new axers. i want ask about recalculate. now i have form parent the name is sales_contractinfo have a field string contract qty and form child the name is sales_contractinfodetail same like parent have a field string contract qty.if the sales_contractinfo had a value and the sales_contractinfodetail there is no value we can add value to sales_contractinfo and recalculate.