[{_id:1,unique_id:1234,config_no:1,configs:[{data:"qwertyuiop"/** random string*/},{data:"asdfghjkl"/** random string*/}]}]

db.collection.aggregate([{$match:{unique_id:1234}},{"$unwind":{path:"$configs",includeArrayIndex:"idx"}},{$set:{"configs.data":{"$cond":{"if":{$eq:["$config_no","$idx"]},"then":"zxcvbnm","else":"$configs.data"}}}},{$group:{_id:"$_id",config_no:{$first:"$config_no"},configs:{$push:"$configs"},unique_id:{$first:"$unique_id"}}},{"$merge":{"into":"collection","on":"_id","whenMatched":"merge"}}])