If you download Unity 5 (in this case, 5.3.1 with the patch), and attempt to open up an old Unity 4.6 project, a pop-up dialogue will prompt you to upgrade to the newest version (which may take a few minutes) and that you will not be able to open the project with an older version of Unity anymore.
You can grab the installer for Unity 5.3.1 here: http://unity3d.com/unityqa/patch-releases
Unity 5.3.1p3 Download Assistant will walk you through the installation: Choose which components you want to install (for example, do you want to download Visual Studio Community 2015 while you're at it?), where to download the files on your machine, and the legal license agreements, etc. You computer may say, "A reboot is pending. Please reboot before starting Visual Studio." if you choose to include VS2015. Click Okay when it's safe to reboot and complete the Unity Setup.
Lots of Unity's features and systems have been upgraded in Unity 5, including Physics, Audio, Animation, Shaders and more. While many areas are upgraded automatically when you open an older project, there are certain areas where you may need to manually adjust or refactor parts of your project.