My iphone is full - HOW DO I DELETE VIDEOS FROM A CELL PHONE

If you downloaded the videos you can delete them from your phone. Likely they would be in the Photo album, maybe in a separate video subfolders.

However, the videos are normally stored in the SS servers, not in your phone. You cannot delete them from the SS servers.

Again though, unless you have actually taken the steps to download videos to your phone, they are not what has your phone filled up.

What whoaru99 said.  There are no videos on your phone unless you went through the trouble of downloading them all which would have taken up a lot of your time and you'd be very aware that you spent hours doing it.  So your phone is probably full of other things.  

A quick Google search found this if you think you need it.

https://www.imobie.com/support/delete-videos-from-iphone.htm