Did you try updating thru iTunes and fail to update or what ?
Install iTunes and restart your computer, if requested.
Connect your iPhone to your computer thru USB charging adapter cable. When asked, give necessary permissions on the iPhone. Then your iPhone will appear in the iTunes window as shown below. At this point, the software update dialog box will open by itself. If it does not open on its own, click on the small phone icon as shown and then click Summary.
Hope this helps.