[{_id:"guild1",users:[{"id":1,"member":"Test"}]}]

db.collection.update({_id:"guild1"},[{$addFields:{users:{"$reduce":{"input":"$users","initialValue":[/** put your to-be object here*/{"id":1,"member":"Adam"}],"in":{"$cond":{"if":{"$in":["$$this.id","$$value.id"]},"then":"$$value","else":{"$concatArrays":["$$value",["$$this"]]}}}}}}}])