Apple have made it very easy to update your iPhone and to help with security they encourage users to have automatic updates enabled.
Why do phones need updating?
The obvious answer is that by updating software you gain access to new features. However, as well as new features the updates help protect you against known security issues.
All software has security vulnerabilities and as manufacturers find out about these they release updates to fix the issue.
To meet security standards all your devices should have the latest security updates installed at all times.
How to Turn on Automatic Updates for IPhone
For iOS 14:
Go to Settings 
> General > Software Update.
Tap Customize Automatic Updates (or Automatic Updates). You can choose to automatically download and install updates.
How to Update and iPhone Manually?
For iOS 14:
Go to Settings > General > Software Update.
The screen shows the currently installed version of iOS and whether an update is available.
How to Update older versions of iOS?
More information about how to update your iPhones can be found on the apple website.