I know it\'s not a big deal to reinstall, but the file is more than 4 GB large; and the installation process is time consuming. When the final VS 2015 is released, can I update
Yes, this is a supported upgrade path and we test for it. You may also have other version of Visual Studio installed, side by side installs are also tested for (VS 2013 side by side with VS 2015 is officially supported).
That being said VS 2015 does bring in a number of external components so there's always a risk that on some configuration the upgrade will not work properly.
Source: I'm working on the Visual Studio team.