Unable to Update Start Date in Geography data which we imported manually

Summary:

We have created Geography structure in Dev1 as per client requirement and based on that structure we have imported the geo data into cloud, but these data were loaded without StartDate into the Cloud.

It automatically took the start date as the system date on which we have imported the file into the system. But we need the start date to be 1/1/1951, so we tried loading with the required start date by adding start date field to the file and tried importing again through 'Import management' but the date is not getting updated.]

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