Database
[{"_id":ObjectId("5e786a078bc3b3333627341e"),"value":{"items":[{"itemName":"alpha305102992","itemNumber":""},{"itemName":"beta305102630","itemNumber":"P5000"},{"itemName":"gamma305102633 ","itemNumber":""}]}}]
Query
db.collection.update({},[{$set:{value:{items:{$map:{input:"$value.items",in:{$mergeObjects:["$$this",{itemName:{$toUpper:"$$this.itemName"}}]}}}}}}])