[{"id":"mongoId","results":[{"score":80,"details":{"credit":2,"creditPoints":40,"creditScore":0}}]}]

db.collection.update({},[{$set:{results:{$map:{input:"$results",in:{$mergeObjects:["$$this",{details:{$mergeObjects:["$$this.details",{creditScore:{$multiply:["$$this.details.credit","$$this.details.creditPoints"]}}]}}]}}}}}])