[{"jk":"some_key_for_query","s":"abcd"},{"jk":"some_key_for_query"},{"jk":"some_key_for_query","s":"abcdwerwer"},]

db.collection.update({"jk":"some_key_for_query"},[{$set:{s:{$cond:[{$gt:[{$strLenCP:{$ifNull:["$s",""]}},5]},"$s","newString"]}}}],{upsert:true,multi:true})