Many iOS users have had this issue before when trying to update their devices and it can be caused by a number of things.
iOS updates come with security fixes, bug fixes, and new features and it is very important to install them when available. However, you may have had this one time where you clicked on Software Update in Settings and no new update popped up. Luckily, this can be fixed, or you can at least find the reason why the update is unavailable.
Solution #1: Patience

Sometimes, when Apple releases a software update, it can take a bit of time for all compatible devices to receive the update. This could mean you simply have to wait a little while before the update is available.
Solution #2: iTunes

You can force updates to your iOS device by using iTunes on a PC or Mac. Simply plug your device in, launch iTunes, click on your device, and then click the button ‘Check for software updates’ next to your device info. iTunes should check for updates and install any found.
Solution #3: Check your connection

A bad internet connection could cause your device not to be able to check for updates. First of all, you cannot use mobile data to update your device and must use a WiFi connection. This means that if you are not connected to WiFi, you must connect to a network before being able to update. Secondly, the WiFi connection must be good. If you only have one or two bars, it is unlikely you will be able to install the update. So make sure you have a good internet connection before installing.
Cause #1: iOS device not supported

Your iOS device will always show the latest updates available for your device. If your device was dropped in a new iOS version, you are unlikely to get any new updates unless there’s a serious bug or security issue. So, if you’re trying to update an iPhone 6 to iOS 13, stop, as you’re wasting your time. Check the list of supported devices in the new iOS version to see if your device is compatible.
Cause #2: Update not actually released to public yet

Before releasing updates to the public, Apple usually release beta builds to those who opt in for them. If you saw a recent news article saying an update has been released and your device is compatible but you just aren’t getting the update, re-read the news article to see if the update released is a beta build. If so, you’ll either have to wait for it to be released publicly or opt for beta builds which may be unstable.
Cause #3: There actually is no update available

If you’ve seen a trailer on YouTube that you didn’t realise was a concept or news from an unreliable website saying a new update has been released you may rush to update your device only to find nothing there. Make sure you check reliable websites for news on whether there has been an update released as it is possible you have fallen for fake news or didn’t see the ‘concept’ in the title of the YouTube video.
I hope this guide helped you update your iOS device or at least find out why you couldn’t. If you have an issue with your device that you need a guide like this for, simply just leave a comment explaining the issue and I will get back to you as soon as I can.