The OnePlus X was essentially the OnePlus 2 mini from last year. It was a smaller device, with lower-end specs (but still very much a flagship, by most standards) and it was available even cheaper than the OnePlus 2. Which was already a steal. OnePlus has since decided that there won't be a successor to the OnePlus X, and that they want to keep their focus on their high-end flagship smartphones, like the OnePlus 3 that they announced earlier this summer. But that doesn't mean that they won't be updating the OnePlus X. In fact, it's seeing an update right now, to Oxygen OS 2.2.2.
Unfortunately, the update does not bring the OnePlus X up to Android 6.0 Marshmallow - which is still a bit sad, given the fact that Android 7.0 Nougat is launching this month. But OnePlus does bring in a slew of changes to the OnePlus X, and a ton of fixes as well. They've upgraded the camera UI to version 1.4, improved camera frame rate performance, updated Swiftkey to v6.3.3 and carrier name mapping, Added July's security patch, Added trusted face unlock security option, and fixed a slew of other issues. So if you've been facing issues with the OnePlus X, this update should definitely provide you with a much better experience.
The update to Oxygen OS 2.2.2 is about 325MB in size, which is pretty hefty, but there are loads of changes here. If you haven't received the update already, you can head to Settings, then tap on 'About Phone' and go to 'System Updates' and tap on 'Check for Updates' and you should be able to pull down the update. But do keep in mind that this is indeed a staged roll out. So you may not see the update right away, and it may be a few days before it actually makes its way to your device. Currently, there's no file available to sideload to your OnePlus X and update it to the latest version of Oxygen OS. It's a shame that it's still not getting Marshmallow, and it's looking like the only way you'll get Marshmallow is by rooting the OnePlus X and flashing a custom ROM like CyanogenMod 13.