Update software
Why I cannot update software my iPhone X?
iPhone X, iOS 12
You can update your iphoneX on the most updated software update 14.4 as it is supported for that.
you just needs to connect your iPhone to WiFi and then go in settings and then go to general and then click on software, now that you have connected your iPhone to WiFi, you can update the software from there,which was black in colour now it will be shown in blue colour and you can download and install it.
hope this will help.