The Google Nexus 5X has got fed up of waiting for the arrival of Android Nougat so it has taken the backdoor route to get the update. It has done this by way of a custom ROM with the name of CM14.1.
CM14.1 is a nightly build from the CyanogenMod team and so there could be some issues with the ROM and some things may not work correctly. Before you do install it you will need to check for at least 80% battery life, the device should be rooted and you need USB debugging enabled. You will also have to have CWM or TWRP installed on the handset.
Download CM14.1 ROM to the computer and Google Apps and then connect the device to the computer and copy the file to the SD card. Then you should switch the handset off and then unplug it.
Head into recovery mode and hold the power, volume up and volume down buttons and when you let them go you will see Fastboot or Bootloader. You can use your volume keys to navigate and then choose with the power button.
You should make a backup and do a Nandroid backup and then go back into the main menu. Wipe data by choosing wipe data/factory reset and you may then wipe the cache partition and delvik cache.
You can find the CM14.1 ROM on your device and install it and confirm it and then do the same for GApps. When you reboot your handset you will have Android 7.1 Nougat installed.