There is no plug-in to install to update Lightroom 6. It's all part of the Lightroom program. Your camera model requires at least Lightroom 6 .5. When you install Lightroom 6 initially you have Lightroom 6 .0. The final update was Lightroom 6 .14. Have you installed the updates you are entitled to?
You can get the 6.14 download at the link below.
Click on the blue chevron icon to the left of Adobe Photoshop Lightroom 6 to expand the list.