To Download a Video that you already have Uploaded, all you need to do is click on the Ellipsis (3 Dots) to open up the sub-menu. You can then select 'Download Video'
A new window will appear and you will be able to choose which resolution you want to download the video in:
NOTE: The 'Download Now' button should work as intended for Microsoft Edge and Safari Internet browsers.
If you're using Google Chrome, right-click on the "Download Now" button and choose "Save Link as..."
If you have Mozilla Firefox, the Download Now button will open the video in a new tab where you must right-click on the video and then click on "Save As".