[{"_id":{"$oid":"62df66945e2ecab37dd56a69"},"AllApplicableSteps":[{"StepName":"Open","StepTitle":"Open","StepNo":1,"StepDesc":"Open Step"},{"StepName":"Pending","StepTitle":"Pending","StepNo":1,"StepDesc":"Pending Step"},{"StepName":"Approved","StepTitle":"Approved","StepNo":3,"StepDesc":"successful"},{"StepName":"Rejected","StepTitle":"Rejected","StepNo":3,"StepDesc":"rejected"}]}]

db.collection.aggregate([{$addFields:{AllApplicableSteps:{$map:{input:"$AllApplicableSteps",in:{"$mergeObjects":["$$this",{StepTitle:"$$this.StepName"}]}}}}}])