The new Rhino version 6 includes grasshopper so you no longer need to install that as a separate plug-in. The Hummingbird components are installed in the same location (C:\Users\<UserName>\AppData\Roaming\Grasshopper\Libraries\Hummingbird) for both Rhino 5 and 6 so, if you have already installed them, they should automatically appear after you install Rhino 6. If you are making a fresh install of Rhino 6, the installation program (GrasshopperSetup.msi) will work exactly as before. (Note that you can have both Rhino 5 and 6 installed at the same time, and the single Hummingbird installation will work for both.)
The Rhino 5 Grasshopper components are upward compatible so they should work exactly the same in Rhino 6. For now, we are not planning any changes to the Hummingbird package so this should not be an issue. There are some new elements in the Rhino 6 version of Grasshopper that are not backward compatible with Rhino 5. This means that at some point in the future the Hummingbird tools will not work with Rhino 5 so it is a good idea to upgrade to Rhino 6 as soon as possible.