Videos are the main asset that exist within a node. If you haven't learned the basics of the Composer, make sure to read our "Getting Started with the Composer" article first.
The video assets that you've uploaded for your narrative in the CMS, will all be in the "Videos" tab that you can find next to the "MiniMap" tab at the bottom of the Composer canvas.
To publish your videos you have to make sure you put them into a node and connect the node with the "User Starts Video" node.
You can change the video that is in a node by right-clicking on the thumbnail image and selecting "Edit Video" and then click "Select an Asset" to change the video asset in the current node.