You need to use what ServiceNow calls Conditional coalesce.
It is basically a Field map where the target field is Sys ID and it is marked as a scripted one.
In the script of such a Field map one needs to write code that looks up what record needs to be updated and
- sets global variable answer to the sys_id of the record found (obviously if found) - in this case that record will be updated -or
- sets global variable answer to -1 if no record is found - in this case a new record will be created.
In your case you would need to write a script that searches the table where fields A and B have the same values as the transformed record and sets global variable answer to that record's sys_id - if found, otherwise sets global variable answer to -1.
E.g. to use your example, if the transform record contains A == India and B == Bangalore, it should set global variable answer to the sys_id of Record:1, if the transform record contains A == France and B == Bangalore, it should set global variable answer to -1, cause no such record exists/will be found.
Do have a look at the docs page referenced above for perhaps clearer examples.
