Hi Samarpita,
The ability to update startDate and/or endDate is not currently exposed to users through Data Loader. As I understand it, this is by design. If this is functionality that you'd like to potentially see added to the product in the future, please submit a request for enhancement for the team's consideration via our Ideas portal. For more info, see: https://docs.reltio.com/en/discover/community--ideas/submit-your-product-ideas