This Service Pack will bring Autodesk AutoCAD Civil 3D 2015 64-bit up to the most current version and includes the most recent fixes for your product. The Readme contains the latest information regarding the installation and the use of this Service Pack. It is strongly recommended that you read the entire document before you apply this Service Pack to your product.
You can download Service Pack 2 using Application Manager (Start>Search for "Autodesk Application Manager"). Ensure you have the latest version of this program and check for updates. You should be able to see something like this: