Iphone software
Iphone software is downloaded but i am not able to install
iPhone X, iOS 12
Assuming you're trying to update your device wirelessly (via wifi), see this support article:--> If your iPhone, iPad or iPod touch won’t update
If the issue persists, if your iPhone is still running the old 12 iOS version, you might have troubles updating it wirelessly. I would try to update it by connecting with a computer running iTunes (PC) or Finder (Mac).
If the issue still persists, make an appointment and ask for help at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector
After checking it out, they'll give you all the available options.
Regards
Giulio