i'd like to install confluence source editor plugin, after click "Get it Now" the confluence-source-editor-1.1.3.jar was downloaded, my question is how to install it?

The best way to do it is forget the download and use the "admin -> add-ons -> find new addons" to find it and add it.  This is the approved route (and it protects you from installing invalid versions of add-ons)

If you want to do it by upload, then there's an "upload add-on" link in the manage add-ons section

Of course, these only apply to Server versions of Confluence.   If you are on the Cloud version, you can only install Connect add-ons.