How can I open raw files? [Unable to open Nikon D810 Raw files in PS CS5]

Your camera requires at least Camera Raw 8.6. The newest version that Photoshop CS5 can use is Camera Raw 6.7. Your free alternative would be to download the DNG converter and create digital negative copies of your NEF files that are compatible with Camera Raw 6.7. The DNG files will have all the same raw data, but in a DNG container. Or, you can purchase Lightroom 6 and add it to your workflow. Or, you can subscribe to the Creative Cloud photography package that includes both Photoshop and Lightroom and pay a monthly or annual subscription fee. And yet another alternative would be to purchase the current version of Photoshop Elements which supports the latest version of Camera Raw. If you want to consider the DNG converter, here is some reading to help you understand what that is all about, as well as links to download the converter, and a tutorial on how to use the converter.

Digital Negative (DNG), Adobe DNG Converter | Adobe Photoshop CC

Adobe - Adobe Camera Raw and DNG Converter : For Windows

Adobe - Adobe Camera Raw and DNG Converter : For Macintosh

Camera Raw: How to use Adobe DNG Converter - YouTube