Modify existing data in table when importing from XML

Hi everyone
I created an XMLPort which imports data from XML to a table in my table.
It only adds new lines in table when importing which is fine until it tries to import an entry from XML which has the same Serial No. (primary key) as an entry in my table when it returns error.
I want to add CONFIRM window and modify existing data.
For example, I have Car Serial No., Model and License Plates fields in my table.
When I import a car with same serial no. but different License plates it returns error since it tries to add new line.
I hope you get what I need
THANKS!