I was unable to update my phone

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

• iOS 15.6 —> 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

Refer to the pic for more...

What iPhone model are you using? What iOS version is currently running on it?

What happened when you tried updating it? What error message(s) are you getting?