2. Please make sure that you first create a full website backup so that you are safe no matter what happens.
3. The oldest version you are able to download from that link is "3.8.3". After that, I recommend that you update to each new major version (3.9, 4.0, 4.1, 4.2 and 4.3).
You can also try to create a full website backup first and then update to the latest version and see what happens - it there's gonna be serious errors and not simple notifications, then please update gradually.
