Apple regularly releases updates with bug fixes and improvements, and once a year, big iOS updates with new features. They are quite easy to install, either from the device "over-the-air" or computer.
In this case, starting from iOS 14.5, it is possible to choose whether to stay on the current firmware and receive only important security updates or upgrade to the current version to have the latest software on the device.
This is useful because, despite older iPhones and iPads supporting modern versions of iOS, experience has shown that installing them is not a good idea. It leads to a drop in performance and battery life, so think twice and study the reviews of other users, for example, by watching reviews on YouTube.
When you install an over-the-air update, a certain amount of free space is required to download the firmware files, and if there isn't enough space, iOS may offer to delete voluminous apps automatically. Don't worry and agree - all information will be re-downloaded after the update. If the resources are not enough, try to free the storage yourself.
If you have a current model or iPhone 2-3 years old, it's easier not to bother and use the auto-update function. In this case, when a new version of iOS is released, the smartphone will independently download and install it overnight, provided that it is connected to a charger and Wi-Fi.
This is also a very simple method. Make sure that the iPhone is at least 50% charged and has a stable connection to the Internet via WiFi or cellular network.
You'll need a Mac or Windows computer with iTunes and an iPhone connection cable to update in this way. This option comes in handy when errors occur during an over-the-air download or when your smartphone doesn't have enough memory.
Connect your iPhone to your computer with the cable, and then open Finder (on macOS Catalina and later) or iTunes (on earlier versions of macOS or on a Windows PC).