Do not modify the structure of the exported data template. Keep the details in the exported template as is. Add-in the required (modified) responses in this template and only add unique identifier of every participant who completed the survey. Do not add any other information. Your import file basically needs to have following 3 points:
1 - Respondent column: True (OR 1)
2 - Unique Identifiers
3- Survey responses
It is not recommended to edit or modify a single response via the import option. Because you can do this directly from the data tab of the project.
I would suggest to add all the collected responses in the import file --- save the file as CSV.
Now first delete your existing responses in the project and then upload the data file that you saved as CSV earlier.
Reference resource: https://www.qualtrics.com/support/employee-experience/creating-ee-project/data-tab/importing-responses-ee/