Like other messaging apps whatsapp too supports deleting already sent messages, but it has a time limit of 2 days. Messages older than 2 days can't be deleted.
But there's a loop hole whatsapp is dependent on your system clock to detect messages older than 2 days.Now if we just change the date and time of clock without letting whatsapp know we can delete messages older than 2 days also.
Below given is a short tutorial explaining the process click on thumbnail to know more

Tutorial