An update delivered either by OTA or by desktop application is just that. What @
samtechtomtom is suggesting will replace the entire system, not update some files.
Here's a YouTube video that explains how to use Smart Switch. It's not my video. This for a Galaxy S6 Edge, but the process is the same. The Smart Switch application has been updated (this video is almost two years old), so it looks a little bit different than in the video. The part that you need is almost the same. In the video, the tab you want is labelled "Software Update and Initialization". Now it's called DEVICE INITIALIZATION.
1. Connect your phone and hit the MORE button on the upper right corner of the Smart Switch window.
2. Click on EMERGENCY SOFTWARE RECOVERY AND INITIALIZATION.
3. Click on DEVICE INITIALIZATION.
4. Click OK and follow the prompts.
The software will grab the latest OS for your phone, put the phone into the correct maintenance mode, and install the OS with the latest update. This should leave your phone as though you took it from the box with Nougat preinstalled. Yes, you'll lose everything, so back up your photos, music, docs etc.