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, then select 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: You can click on the Download  ↓  icon to download the video, which will activate once the rendering is complete.

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 video on the left will be the latest version).

Note: A video can only be exported on a paid plan.