Is it possible to update list datatype (from string to date) via API call?

Hi Harikrishan2806​  , you can do it using this endpoint 

https://contoso.sharepoint.com/sites/siteName/_api/web/GetList(LIST_ID)/Fields/GetByInternalNameOrTitle('COLUMN_NAME')

MERGE request and SchemaXml property.

{"SchemaXml":"<Field DisplayName='COLUMN_NAME' FriendlyDisplayFormat='Disabled' Format='DateOnly' IsModern='TRUE' Name='COLUMN_NAME' Title='COLUMN_NAME' Type='DateTime'></Field>"}