@Poorvi Sachar, you can use the following REST API to update an entity attribute for HDP 2.6 or HDP-2.5:
HDP-2.6 with Atlas 0.8:
curl -u admin:admin -X PUT 'http://localhost:21000/api/atlas/v2/entity/guid/<column_guid>?name=comment' -H 'content-type: application/json' -d '"new comment"'
For HDP-2.5 with Atlas 0.7:
curl -u admin:admin -X POST 'http://localhost:21000/api/atlas/entities/<column_guid>?property=comment' -H 'content-type: application/json' -d 'another comment'