Database
[{"_id":"objId1","states":[{"place":"EU","skId":"SK01"},{"place":"IN","skId":"SK05"},{"place":"NZ","skId":"SK08"}]}]
Query
db.collection.aggregate([{$match:{_id:"objId1"}},{"$addFields":{"input":{"place":"KL","skId":"SK08"}}},{"$addFields":{"states":{"$map":{"input":"$states","as":"s","in":{"$cond":{"if":{$eq:["$$s.place","$input.place"]},"then":"$input","else":"$$s"}}}}}},{"$addFields":{"states":{"$setUnion":["$states",["$input"]]}}},{"$project":{_id:1,states:1}},{"$merge":{"into":"collection","on":"_id","whenMatched":"replace"}}])