Visual Studio 2015 Update 1 have changed the main Visual Studio icon (which you see on the taskbar):

Visual Studio 2015 RTM icon

Visual Studio 2015 Update 1 icon
The intent was to make it different from the Visual Studio 2013 icon, but the result is not obviously better. In this article I will describe how you can set your own icon for Visual Studio.
If you have Visual Studio 2015 pinned to the taskbar, hide labels on taskbar buttons and just installed Update 1, you don’t see the new icon. The taskbar icon remains the RTM one.
For a new Visual Studio 2015 Update 1 installation or not pinned taskbar icon, the icon will be the new one. To change it, first unpin Visual Studio 2015 from taskbar if it is pinned. Then run VS 2015, right click on the VS taskbar icon, right click on the Visual Studio 2015 entry and select Properties:

Taskbar context menus
In the Visual Studio 2015 Properties dialog, click Change Icon, Browse for example to the VS 2015 RTM icon and save changes:
When you restart Visual Studio, it will show your selected icon on the taskbar and you can pin it back if you want:
If your taskbar is configured to show labels, a running Visual Studio 2015 taskbar icon will always show the default Update 1 icon:

Labels on taskbar buttons
You can install the Set original Visual Studio 2015 RTM icon extension for Visual Commander to change the running VS icon with a label. By default, it downloads the RTM icon from my site, you can copy it locally to your %AppData% folder or substitute for any other icon:
Finally, it you like VS 2015 RTM icon, but want to differentiate it from VS 2013, you can install and customize Add version overlay to the Visual Studio 2015 taskbar icon extension for Visual Commander:

Version overlay for Visual Studio
Go ahead and make Visual Studio 2015 looks the way you want it to look!
Update (January 26, 2016): Vishwanath Uppala published a VS 2015 icon in a different color.

Custom Visual Studio 2015 icon