Copy values between tables via action

How to get the value from one table to another table with an action?
I have a simple “form view” table where i can choose product and set the quantity for it.
I want this quantity to be copied over to multiple rows in another table if the “product=product”.
It would be easier to do it in the same table but i need to choose the product somehow, which means i need
to fill up a form and the form creates a new row that i do not need.

ACTIONS - DATA: ADD A NEW ROW TOO ANOTHER TABLE

I do not want to add a new row.
I need to change a value in an existing row with an action.