You cannot change it.
If the message type is SMS/MMS, the limit is set by your carrier. All the USA carriers use really small limits like 600-1000 KB.
If you are using an instant message service, like WhatsApp, their limits are much bigger but still limited.
To get past the limits, you need to use a photo hosting site or a file sharing site to upload the video to, then send the URL of the video in your message.