You would need the external ID of task to import data to an existing task, in the absence of this a new a task would be created.
Would suggest to export the tasks to update with external ids and then use that spreadsheet to import
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Updated Answer
Believe it might be easier to import in the mail.message model to update the comments on a task.
Settings > Techinical > Messages
Since these are new comments the fields you may want to have on your import file
Author
Date
Related document ID - The task ID of the task where the comments are being updated
Related Document Model - Project.task
Subject - If you wish to have one else leave blank
Type - Comment
Subtype - Note
Contents
Hello Jaideep,
Thanks for your answer. So, if I set the ID, will it import the comments to the messages area, and not to the description, right?
Have updated my answer, was getting error while trying to test import on the project.task model. To link comment to a task from messages you would need the document ID, you should be able to export the Task ID (document ID) from project/ all tasks (field is ID)
Hope this helps