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 Google Chrome, Microsoft Edge, and Safari Internet Browsers. If you're using Mozilla Firefox, the download now button behaves differently. The button will open the video in a new tab where you must right-click on the video and then click on "Save As".