How can I update data to a specific row of a Zoho CRM subform from a source subform?

How can I update data to a specific row of a Zoho CRM subform from a source subform?

I have subform1, with 4 rows(example: A,B,C,D) and subform2 with 3 rows(A,B,D). I want to update the Subform1 with the sum of the row A from subform1 and Subfrom2. I want to update only the row A. However, When i update it, I see only the rows A, B and D and not C. Since I updated the subfrom2 to calculate it. 

I am not sure how to update a specific row in a CRM subform