User u = [select Id from User where Profile.Name='System Administrator' Limit 1];

    String query = 'SELECT Id,Name,Status ,OwnerId  FROM Lead WHERE Status != 'Closed - Converted' AND CreatedDate < LAST_N_DAYS:30';
    return Database.getQueryLocator(query);
}

global void execute(Database.BatchableContext BC, List<Lead> scope)
{
     for(Lead le : scope)
     {
         le.OwnerId = 'u.Id'  ;
     }
     update scope;
}
global void finish(Database.BatchableContext BC)
{
}