[{"_id":ObjectId("62c2e94e65f32725f8f62b79"),"updatedAt":ISODate("2022-06-29T13:10:36.659Z"),"createdAt":ISODate("2022-06-29T08:06:51.264Z"),userID:1,myImage:"imageC",images:["imageA","imageB","imageD","imageE","imageF"]}]

db.collection.update({userID:1},[{$set:{images:{$reduce:{input:"$images",initialValue:[],in:{$concatArrays:["$$value",{$cond:[{$eq:[{$size:"$$value"},2]},["$myImage","$$this"],["$$this"]]}]}}}}}])