Direct link is https://software.cisco.com/download/release.html?mdfid=286304360&flowid=&softwareid=284272933&release=3.1.6&relind=AVAILABLE&rellifecycle=&reltype=latest
The Software Update checker inside Prime Infrastructure sees it, as expected.
Please take a few minutes to review the Release Notes.
http://www.cisco.com/c/en/us/td/docs/net_mgmt/prime/infrastructure/3-1-6/release/notes/cpi_rn.html
Please know that this upgrade is nearly 1 GB, and that it will take more than 15 minutes to install–please be patient.