There is an update for the preview of Visual Studio 2013, this was release a couple days ago. There are 2 ways to get the update.
The first look at the notifications in VS
The other way is to browse to Microsoft Download Center using the link below
Once you have downloaded the exe file, the installation is the same as the updates for VS 2012, also remember if you are wanting to install this update on multiple machines you can download all the files using the /layout command argument (see below) and then copy the files to each machine.
Open cmd.exe and navigate to the folder where you downloaded the exe update file to, then run “VS2013 Preview Update.exe” /layout
This will launch the update application prompting for a location to download the installation files to. Next Click Download and wait until the download completes.
Now that the files are complete launch the installer again without /layout and then the installer will use the downloaded files instead of downloading each file each time you run the installer.
This method can be used with all VS bootstrap installers and is not only a VS 2013 installer feature.