I assume what you mean is how do you redownload previously purchased videos?This depends on which country you are located in and what videos you are attempting to redownload.
Some items you’ve previously purchased are not eligible for redownload via iTunes in the Cloud and thus, will not appear on your Purchased page. This content includes ringtones, audiobooks, movie rentals, items that have been modified or removed from the store since you purchased them, anything that may have been refunded, as well as certain movies.
For some movies, you will see the message "When purchased, this movie will not be available from iTunes in the Cloud” if the movie can't be downloaded again. This message appears under the Buy button on the iTunes Store product page. This is why you should always back things up.
To learn what previously purchased content is available for download in your country, visit this article:
iTunes in the Cloud and iTunes Match Availability