[{"_id":1,"myarray":[1,2,3]},{"_id":2,"myarray":[1,2,3]}]

db.collection.update({"_id":{"$eq":1}},[{"$addFields":{"myarray":{"$map":{"input":"$myarray","as":"m","in":{"$cond":[{"$gte":["$$m",3]},"UPDATED!","$$m"]}}}}}])