[{"_id":1,"item":"sweatshirt","price":{"INR":45.99},"currency":"INR"}]

db.collection.aggregate([{$replaceRoot:{newRoot:{"$mergeObjects":[{"$arrayToObject":{$map:{input:{$filter:{input:{"$objectToArray":"$price"},cond:{$eq:["$$this.k","$currency"]}}},in:{k:"pricenew",v:"$$this.v"}}}},{_id:"$_id"}]}}}])