[{"users":{"assigned":[{"firstName":"naveen","lastName":"bala",},{"firstName":"SHAJU","lastName":"HARI",},{"firstName":"PADMANESH","lastName":"NC",}]}}]

db.collection.aggregate([{$set:{"users.assigned":{$map:{input:"$users.assigned",in:{firstName:{$concat:[{$toUpper:{$substrCP:["$$this.firstName",0,1]}},{$toLower:{$substrCP:["$$this.firstName",1,{$strLenCP:"$$this.firstName"}]}},]},lastName:{$concat:[{$toUpper:{$substrCP:["$$this.lastName",0,1]}},{$toLower:{$substrCP:["$$this.lastName",1,{$strLenCP:"$$this.lastName"}]}},]}}}}}}])