Update existing node (not created by Migrate) via Migrate 8.5.x
Updating existing content was possible by changing the default behaviour of a migration with the system of record concept in Drupal 7.

In Drupal 8 this concept has been revised &amp; is available with the overwrite_properties option in the destination of the migration YAML:

...
destination:
  plugin: 'entity:node'
  # define the entity properties that are to be updated.
  overwrite_properties:
    - title
    - field_foo
    - field_bar
...