replacing article or basic page image with video
Here is a standard front page. The teaser view. I think this is what you see and where you would like to see the video instead of the image.





This seems like what you would like. On the preview, add the video.





This is the full node view. As you can see, the full text is here.





When you go to the content type admin/structure/types/manage/article/display the default display is the full node view





On the teaser display, /admin/structure/types/manage/article/display/teaser (this is for the article content type). At the top right it says teaser.  


Here you need to add the "Embed field". By default it was hidden.

You can move the image to hidden if you want.



And with this you get the image from the second screenshot.

I think that's what you want/need. 



Original answer. Updated above based on comments.

If you want to upload, instead of embed, the video module has the option,

https://www.drupal.org/project/video