[{"key":1,"Array":[{"Prop1":1,"Prop2":2},{"Prop1":2,"Prop2":3},]},{"key":2,"Array":[{"Prop1":1,"Prop2":2},{"Prop1":2,"Prop2":3},]}]

db.collection.update({},{"$set":{"Array.$.Prop1":""}},{"multi":true,"upsert":false})