It is possible to update Android version in Honor 5X to Nougat.
You have two options in this case.
Install CM14 on Honor 5X
Cyanogen Mod is officially deprecated. And it will be no longer supported. If you are techie and know how to use phone with deprecated ROM then this can be one good option.
1. Download CM14 from the git repository of Cyanogen.
2. Boot into root,backup your device and then wipe the disk.
3. Factory reset the device and then in recovery console access "update.zip".
4. In recovery console, start the installation through the menu.
5. Let the system update and install the ROM.
This method is a bit cluttered now considering any server calls to cyanogen will be called off. And you may have plenty of broken notifications.
Update to Nougat 7 using default Android ROM
This ROM can be downloaded through default android website or the XDA developers. Here are the steps that you can use to update the Honor 5X to nougat update.
1. Download the update to nougat from here :
2. Download the update on device SD card as well as on PC (just to be safe).
3. Save the update with the filename update.zip.
4. Reboot into the Honor 5X recovery into TWRP recovery mode.
5. Inside the recovery mode, wipe the data but before that backup if you have any files.
6. Now go to install option.
7. Navigate to the zip file you just downloaded in SD card.
8. Start the installation.
9. After completing the installation, reboot your device.
Do understand the fact that your warranty will be void after installation through either of this method. Another thing is that you may find that your default call to honor's server won't be processed from now on. Instead you'd be getting further patches from the ROM company that you have installed. Unless you are into experimentation and don;t mind bricking your device, better stick with the old update on the Honor 5X.