You can only download a video that has been exported. 

To download a video that you are editing or have not exported before:

Step 1: Click on the 3 dots of the video you would like to download, and click on the 'Edit' option to enter the project

Step 2: Now click on the 'Download' button, then the 'Export' option, and the video will start rendering. Please wait for the rendering to complete.

Step 3: The Download button will be activated as soon the rendering is completed. You can then click it to download the video.

To download a video that you have already exported before:

Step 1: Go to the 'My Projects' page.

Step 2: Click on the project folder for the video you would like to download

Step 3: Click on the download icon to download the video (the left most video is the latest version)