Import does not UPDATE existing records

Import does not UPDATE existing records

When I import, although I have the original Record Id, from a previous export, the import creates NEW records.
I just want to update those records with the new values from a CSV

I have 1200 leads
I exported 3 fields to CSV. So I have 1200 rows the my 4 fields
  1. Record Id
  2. Last Name
  3. URL field
I formatted the "URL field" as I needed it. in the CSV

When I run the import, I have nowhere to tell importer to UPDATE and not create.
I proceeded as I guessed since the ID was there it was to update the existing record based on this.

I ended up with 2400 records. duplicates

how can I ensure the importer will UPDATE my records rather than create new ones?