Q: how do I backup videos off my iPhone 6 plus

Are you referring to importing your videos taken with the iPhone and putting them on the computer, or are you asking about something else? What computer OS are you using?

Hi suzred1987,

This article goes over backing up all your data: About backups in iCloud and iTunes - Apple Support

If you're wanting to backup just your videos to a PC, this article will be helpful: https://support.apple.com/en-us/HT201302