Database
[{"_id":ObjectId("62c2e94e65f32725f8f62b79"),"updatedAt":ISODate("2022-06-29T13:10:36.659Z"),"createdAt":ISODate("2022-06-29T08:06:51.264Z"),userID:1,images:["image0","image1","image2","image3","image4"]}]
Query
db.collection.update({userID:1},[{$set:{firstItem:{$arrayElemAt:["$images",2]},secondItem:{$arrayElemAt:["$images",0]}}},{$set:{images:{$reduce:{input:"$images",initialValue:[],in:{$concatArrays:["$$value",{$cond:[{$eq:[{$size:"$$value"},0]},["$firstItem"],{$cond:[{$eq:[{$size:"$$value"},2]},["$secondItem"],["$$this"]]}]}]}}},firstItem:"$$REMOVE",secondItem:"$$REMOVE"}}])