Database
[{"key":1,"array":[1,2,3,4,5]}]
Query
db.collection.aggregate([{"$set":{"array":{"$map":{"input":{"$range":[0,{"$size":"$array"}]},"in":{"$cond":[{"$eq":["$$this",2]},100,{"$arrayElemAt":["$array","$$this"]}]}}}}}])