Create an Event in your child form which the Main form handles:
https://msdn.microsoft.com/en-us/library/ms172877.aspx?f=255&MSPPError=-2147217396[
^] - when your child wants to update the main form, it raises the event.
Add a property to the child to return the data that you want you update with.
In the main form event handler, access the property via the instance of the child form - either by saving the instance when you create it in the code you show above, or via the sender parameter to the handler method. The main form then updates itself with the new data.