I what to update my iphone to iOS 14

If you have an iPhone 6s, as indicated in your profile, you can update it, however not to iOS 14. You can only update it to the latest iOS, which at this time is iOS 15.4. See this, Update your iPhone, iPad, or iPod touch - Apple Support

If your iPhone is telling you it is up to date, then you do not have an iPhone 6s, you have an iPhone 6.

These are the iOS options based on iPhone Model --> You need to Identify your iPhone model - Apple Support

• iOS 15.4 —> 6S / 6S Plus, SE (1st Gen and 2nd Gen), 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max, iPhone 13, 13 Mini, 13 Pro, 13 Pro Max
• iOS 12.5.5 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, 

Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

That is not possible. iOS 14 has not been current for a long time.

Either you have an iPhone 6S or newer and may update to iOS 15.4, or (much more likely) you have an iPhone 6 or older which cannot even use iOS 13.