[{Company:"Alpha ",Products:[{ProductName:" Bike",Release_Date:"",Sales:{Today_Sales:[{Date:"...",Sales:12},{Date:"...",Sales:34}],Total_Sales:[{Date:"...",Sales:12},{Date:"...",Sales:34}]},}]}]

db.collection.update({},[{$set:{Products:{$map:{input:"$Products",in:{$mergeObjects:["$$this",{totalSales:{$arrayElemAt:["$$this.Sales.Total_Sales.Sales",-1]},}]}}}}}])