These links show you how to install SI 2015.
Installing the SI 2015 Server
Installing Visio Optional. If you will be using any supported third-party software, we recommend installing Visio, AutoCAD, and QuickBooks on the all machines prior to installing the SI 2015 Client.
Installing an SI 2015 Client
Registration
Upgrading from SIX 2013
Updating SI 2015
SI 2015 Release Notes