How to Update a field value from one data source to another.

Hi everyone,

    I have created a field in purchase order header where in if i type something the system should pop up a dialog saying "Do u want to update this value in PO lines ?" if i choose yes then system should update the same value from PO header to the PO line .. how am i suppose to achieve this ? 

Thank you in advance !!

P.S - I have created the fields in PO header and PO lines.