[{id:1,balance:10,transactions:[{balance:10,transaction_amount:10}]}]

db.collection.update({id:1},[{"$addFields":{"txnAmt":20}},{$addFields:{balance:{$add:["$balance","$txnAmt"]}}},{"$addFields":{"transactions":{"$concatArrays":["$transactions",[{balance:"$balance",transaction_amount:"$txnAmt"}]]}}},{"$unset":"txnAmt"}],{multi:true})