If you want to stay with Lightroom 5 then you will have to convert your NEF files to the DNG format using the free standalone DNG converter available from Adobe. There will be no more updates to Lightroom 5.
If you don't want to buy LR6, you can get the latest DNG converter 9 for Windows or Mac from the following link. The DNG files will then work in LR5 (keep your originals as backups and for use in the camera manufactures software)
To download click here - DNG Converter 9.0
You can convert a whole folder of raw images in one click. See this quick video tutorial: